神经纤维产生动作电位的频率决定于()
A．超常期长短
B．相对不应期长短
C．低常期长短
D．绝对不应期长短
E．整个动作电位时程长短
参考答案：D
Aunt Li ________ the babies very well while their parents were away. [ ]
A. looked at
B. looked like
C. looked up
D. looked after
简述棉花的花芽分化。
