Last week,  Susan and David,  ________our university's student association,  met the Chinese guests

at the New York airport. [ ]

A. to represent  

B. representing

C. represented  

D. having represented

表皮内的树枝状细胞包括（）

A.黑素细胞

B.角质形成细胞

C.朗格汉斯细胞

D.巨噬细胞

E.Merkel细胞