按照巴普洛夫的说法，狗听到铃声吐唾液属于（）
A.无条件发射
B.条件发射
C.生理反射
D.强化反射
E.以上全不是
参考答案：D
什么是自助交费？
I must be off now. My friends _____ outside. [ ]
A. wait
B. have waited
C. are waiting
D. were waiting
