Problem: How does the sentence end?

We had to spray the vines with insecticide, but not the flowers, since the 
OPTIONS:
- vines were impervious to insects.
- flowers were impervious to insects.
Answer: flowers were impervious to insects.

Problem: How does the sentence end?

Christopher got to the store faster than Donald because 
OPTIONS:
- Christopher rode on a horse instead of walking.
- Donald rode on a horse instead of walking.
Answer: Christopher rode on a horse instead of walking.

Problem: How does the sentence end?

Joel was more prepared than Robert, so 
OPTIONS:
- Joel didn't do as well on the science test today.
- Robert didn't do as well on the science test today.
Answer: Robert didn't do as well on the science test today.

Problem: How does the sentence end?

James was giving a lecture on his dog when he was told to round up because of time. The 
OPTIONS:
- lecture is short.
- time is short.
Answer:
time is short.