Official Explanation:
Answer C
Classification: Verb Tenses
Snapshot: This problem illustrates the correct use of the future perfect tense.
This problem demonstrates the correct use of the future perfect tense. Choices A and B, by employing the construction “have reached,” offer incorrect versions based on the present perfect tense. Choices D and E create erroneous alternatives by commingling past tense constructions with those in the future tense. Choice D presents an incorrect version which doubles up the present perfect tense “have reached” with the future perfect tense “will have backpacked.” Choice E mixes the past tense “reached” with the future perfect tense (in the progressive form). For the record, an equally correct answer would have been: “By the time we reached France, we had been backpacking for 12 weeks.” This would represent the correct use of the past perfect tense. Of course, the original sentence clearly indicates that the travelers are looking into the future—they have not yet arrived in France.
The future perfect tense and the past perfect tense are very much opposite in terms of time frame but structurally similar. Here are two more ways to draw a distinction between these two tenses: Past perfect tense: By the time something happened (second event), something else had already happened (first event). Future perfect tense: By the time something happens (second event), something else will have already happened (first event).