题目

 –How long ______ the book be kept?         --For two weeks, but you ______ return it on time.         A. can; may       B. may; need       C. can; must      D. must; need 答案：C如图，在平行四边形ABCD中，AE⊥BD，∠EAD=60°，AE=2cm，AC+BD=14cm，则△OBC的周长是_______cm．