∵4-2x有意义，
∴4-2x≥0，
解得x≤2．
故选C．
—Can you speak Japanese?
—No, I _______. [ ]
A. mustn't
B. can't
C. needn't
D. may not
激活补体能力最强的Ig是（）
A.IgM
B.IgG
C.IgA
D.IgD
E.IgE
