常用于诊断胰腺癌及某些消化系统肿瘤的标志物是（）
A.AFP
B.PSA
C.CA19-9
D.CA125
E.CA15-3
参考答案：C
We have been looking at houses, but haven’t found _____ we like yet.
A．one
B．ones
C．them
D．it
