GWD 2-13不理解

Plants are more efficient at acquiring carbon than are fungi, in the form of carbon dioxide, and converting it to energy-rich sugars.

1. Plants are more efficient at acquiring carbon than are fungi
2. Plants are more efficient at acquiring carbon than fungi
3. Plants are more efficient than fungi at acquiring carbon
4. Plants, more efficient than fungi at acquiring carbon
5. Plants acquire carbon more efficiently than fungi

B和C有什么区别啊，为什么选C?选A为什么不对，不是谓语尽量补出么？

还是不太理解.究竟什么时候该补,什么时候不该补呢?