gwd-11-13 急。。。

Today’s technology allows manufacturers to make small cars more fuel-efficient now than at any time in their production history.

1. small cars more fuel-efficient now than at any time in their
2. small cars that are more fuel-efficient than they were at any time in their
3. small cars that are more fuel-efficient than those at any other time in
4. more fuel-efficient small cars than those at any other time in their
5. more fuel-efficient small cars now than at any time in

 answer C

我选d   请教为什么？

more fuel-efficient small cars than those at any other time in their

==》1. their 无指代

         2. more fuel-efficient small cars 有歧义。到底是更有效率的车呢，还是更多有效率的车？

thanks a lot