gwd 6-39，比较题跪求详解

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

答案是c。我还是不太分得清c和b的区别，有没有高人指点下比较的要点啊？我每次都会错，不知道哪该省略，哪不能省。b为什么不对啊

顺便问一下，A是错在第二个are上吗？如果改为is呢？