So popular ________ that the young generation are dying for one. [ ]

A. iPhone 4 has become

B. iPhone 4 had become

C. has iPhone 4 become

D. had iPhone 4 become

关于现浇混凝土梁式楼梯，说法错误的是（）。

A.常用于层高大的公共建筑

B.由斜梁、踏步板组成

C.传递荷载的顺序为：踏步板一斜梁一休息平台梁一墙体或柱子

D.当荷载或梯段跨度不大时，采用梁式楼梯比较经济