can be 和is to be

Greater efforts to increase agricultural production must be made if food shortage ____avoided.
A can be B is to be
答案是B，但A在语法上没错吧，是不是因为B更好

be able to和can can 和 be able to be able to 和can区别 be able to...和can 的区别??????~~~~ can 和 be able to 的区别 be able to和can的区别 比较can 和be able to CAN 和BE ABLE TO 的区别 can和be able to的异同 will be和 is going to be的区别