The desk is not dirty. You _______ clean it.
A:mustn't
B:shouldn't
C:needn't
D:can't
下列属于条件反射的是( )
①画饼充饥 ②老马识途 ③鹦鹉学舌 ④杯弓蛇影 ⑤公鸡报晓 ⑥大雁南飞