28. The reason for his success is ______ he worked very hard.
A.why
B.that
C.whether
D.How
B
所有T淋巴细胞的表面标志().
A.CD2
B.CD3
C.CD19
D.CD80
E.CD4
地方性斑疹伤寒的传播媒介是()
A.蚊
B.鼠蚤
C.恙螨
D.蜱
E.人虱