14. If Frank ........ us, we wouldn't have finished the project on time.

This is a third conditional sentence, referring to a hypothetical past situation and its consequence.

The structure is 'If + past perfect, would have + past participle'. However, the options provided are simple past or past continuous.

Considering the options, 'hadn't helped' fits the hypothetical past scenario where Frank's lack of help led to the project not being finished on time.