Tenses Test - 7

Because of the presence of the word 'if' we can identify this sentence as a first conditional sentence which is used to express a situation in which the outcome is likely (but not guaranteed) to happen in the future on a condition. It is important to remember that we use the simple present tense in the if-clause and simple future tense in the main clause—that is, the clause that expresses the likely outcome.

Since this an example of a third conditional sentence, which is used to explain that present circumstances would be different if something different had happened in the past, we use the past perfect (i.e., had + past participle) in the if-clause and the modal auxiliary (would, could, should, etc.) + have + past participle in the main clause. According to this rule, option D is the correct answer.

The events mentioned in the given sentence take place at the moment the speaker speaks. The line "I am taking a taxi" suggests the same.
'Am taking' is in the present continuous tense. The blank, too, indicates a continuous action during the present moment. Hence the tense of the verb to be filled will also be in the present continuous tense.
Option B: 'Is being repaired' is in the present continuous tense. Hence it's the most suitable option.
(Also, the verb 'is being repaired' is passive, because the subject 'car' is receiving, and not performing the action 'repaired'.)
Hence option B is correct.
Option A: 'Was been repaired' is grammatically incorrect. 'Been' suggests a perfect tense whereas 'was' suggests a continuous tense.
Option C: 'Was repaired' is in the simple past tense, but the action is taking place in the present, as explained above.
Option D: Similarly, 'had been repaired' is in the past perfect tense. So, options C and D are incorrect.

In formal situations, we can use should + subject (s) + root verb (v) instead of if. Here, the structure will be should + machine + fail, and the main clause usually takes on the simple present tense (ring). Hence, Option A is correct. The rest of the options do not follow this structure, hence incorrect. 

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.