食管癌最常见的死因（）。

A.梗阻严重，不能进食，衰竭

B.穿孔，发生相应并发症

C.大出血

D.转移到肝脏

E.转移到咽隔

The letters for the boss________on his desk but he didn’t read them until three days later.                                                             (2012·天津，2)

A．were put

B．was put

C．put

D．has put