第20题: [单项选择]人体的白细胞上有几类抗原 A. 两类 B. 三类 C. 四类 D. 五类 参考答案:B 答案解析:人体的白细胞上有三类抗原：一为红细胞血型抗原；二为白细胞自身特有抗原；三为与其它组织共有的,也是最强的同种抗原即人类白细胞抗原(humanleucocyteantigen简称HLA)。
第22题: [单项选择]Before a baby babbles his first "Ma" or "Papa", ______. A. he must be exposed to the language B. his mind must have some input of sounds and words C. he learns a lot of utterances D. his mind is busy sorting out the sounds and shapes of words 参考答案:D
第23题: [单项选择]______ she realized it was too late to go home. A. No sooner it grew dark than B. Hardly did it grow dark that C. Scarcely had it grown dark than D. It was not until dark that 参考答案:D 答案解析:No sooner than为固定搭配，但句子需倒装，A错。Hardly和Scarcely须和when搭配，B、C两项错。
