胸腺X线消失的年龄是()
A．新生儿
B．3～4岁
C．5～6岁
D．青春期
E．青春期后
参考答案：B
To keep healthy,Professor Johnson cycling as a regular form of exercise after he retired.
A. took up
B. caught on
C. carried out
D. made for
传播过程的直线模式的缺陷与不足可以概括为（）与（）。
