.The maths problem can be______. A.easy worked out B.easy to

.The maths problem can be______. A.easy worked out B.easy to be worked out C.easily worked out
.The maths problem can be______.
A.easy worked out B.easy to beworked out
C.easily worked out D.easily to work out
为什么选c，work out 不是可以主动表被动嘛 ？
例如这题就选a,This problem is really hard_______ . A. to work out B. to be worked out C. working out D. worked out
谢谢解答