Tenses Test 5

In the given sentence, 'human beings' lives' is a plural subject, as 'lives' is the main noun that receives the action 'prolonged' ('receives', because the sentence is in passive voice).
Option A: 'Has been' refers to an action that has just completed. Also, the auxiliary 'has' refers to a singular subject, whereas the latter is plural.
When the subject is singular, the verb must be singular. The same goes for the plural subject and verb.
Option C: Similarly, 'is' in 'is being' is also a singular subject.
Hence Options A and C are incorrect.
Option B: 'Will have been' is in the future perfect tense. It indicates that an action will be completed in a particular time in the future. While using this tense, the time or date is mentioned in the sentence: 'By 2050, natural fuel will have been exhausted.'
But in the question, there is no specific time mentioned.
Hence option B is incorrect.
Option D: 'Will be' refers to the simple future, and reports an action that is yet to happen, in the future. It's the most suitable option, hence D is correct.

The given statement has two actions mentioned. Both form a sequence of tenses. It is preferable that both tenses are the same. There are various rules and exceptions applied when two past actions are mentioned.
For example, a past tense in the principal clause may be followed by a past tense in the subordinate clause.
Option C: 'Was used' and 'had told' are both in the past tense. The former is in the simple past tense (passive voice) and the latter is in the past perfect tense (active voice).
The passive voice indicates that the subject "lie detector" receives the action.
The active voice indicates that the subject "servant" performs the action.
Hence option C is correct.
Option A: 'Is being used' is in the present continuous tense and 'tell' is in the simple present tense. Both belong to the same tense, but 'tell' is not used in the third person. The correct form must be: tells.
Hence A is incorrect.
Option B: 'Will be taken' is in the simple future tense. But, 'telling' does not indicate a tense. 'Telling' is a participle, which requires an auxiliary to help determine its tense in the sentence. There isn't any auxiliary (helping verb) provided. Hence B is incorrect.
Option D: 'Was employed' is in the past tense whereas 'tells' is in the present tense. The tenses are not the same. Hence D is incorrect.

Here, 'will have crossed' indicates the use of future perfect tense. It is used to indicate an action that will be complete before another event takes place.
Future Perfect Continuous (will have been crossing) is used to indicate an action that will have happened for some time and will not be complete yet at a certain point in the future. 
Simple Future (will cross) indicates an action, condition or circumstance which has not taken place as yet.
Future Continuous (will be crossing) indicates what will be going on in the near future.

The given sentence is a conditional sentence with the conjunction, 'if'. The clause with the word, 'if' is referred to as the 'if clause'. The other clause is called the main clause. The action in the 'if clause' happens first. In case that happens, then the action in the main clause will happen. 

The line "When I come home from school" states an action that is yet to happen, even though the verb 'come' is in simple present tense. ('When' indicates 'the time that' the action will happen).
Option C: 'Will be sitting' is in the future continuous tense. It states an action that will be continuing at a given point of time in the future. This meaning is relevant to the given sentence. As a result, it means: 'The time (in the near future) when I come home from school, I can say for sure that my dog will be sitting at the door..."
Hence option C is correct.
Option A: 'Would been sitting' is grammatically incorrect. 'Been' refers to a continuous tense. It states an action that was or is continuing, whereas 'would' refers to a future event. ('Would' is the past tense form of 'will' in the indirect speech.) Hence A is incorrect.
Option B: 'Has been sitting' is in the present perfect continuous tense. It states an action that began sometime in the past and is still continuing. The given sentence states an action that has not yet happened. Hence option B is incorrect.
Option D: 'Will have sitting' is grammatically incorrect. The auxiliary 'been' is missing: will have been sitting.
Hence option D is incorrect.