By the time the plane reaches Harare, it … in the air for twelve hours

Analyze the question

The sentence is: "By the time the plane reaches Harare, it __ in the air for twelve hours."

It uses "by the time" + a present tense verb (reaches), which refers to a future point. The gap must show a duration of time that will be completed by that future point.

Explain the concept

When we say "by the time + [present simple]", the main clause normally uses the future perfect (will have been / will have + past participle) to show that something will be complete or have lasted a certain length by that future moment.

Example: "By the time he arrives, I will have finished the work."

"Would have been" is the past-tense form of "will have been". It is the standard way this structure appears in exam-style questions when a future-in-the-past or hypothetical reading is accepted.

Test the options

• A. has been: Present perfect. It does not fit with a future time reference ("by the time the plane reaches..."). ✗
• B. would be: This does not express completion or duration up to a point ("for twelve hours"). ✗
• C. would have been: This is the perfect form, expressing a duration completed by that point. It is the closest match to the future perfect needed here. ✓
• D. should be: This expresses obligation or expectation, not duration. ✗

Answer

The sentence becomes: "By the time the plane reaches Harare, it would have been in the air for twelve hours."

Note: Strictly, "will have been" is the ideal form, but it is not offered. Among the options, only C has the perfect structure needed, so it is the best choice.

Exam tip

When you see "by the time" or "by + a future time" with "for + a period", look for a perfect form (have been / have + past participle). Eliminate options with no "have", because they cannot show completed duration.