— What happened _____ your father?
— He had an accident.[ ]
A. on
B. in
C. to
D. at
答案:C
参与Ⅰ型变态反应的抗体是()
A.IgE
B.IgG
C.IgM
D.IgD
E.IgA
如图所示,在同一平面内有六个共点力,大小分别为F、2F、3F、4F、5F、6F,相邻的两个力之间的夹角均为60°,求它们合力的大小和方向。