第一恒磨牙最适宜作窝沟封闭的时间是
A．6～7岁B．7～8岁C．8～9岁D．9～10岁E．10岁以后
参考答案：A
He often goes to work ______ subway. [ ]
A. by
B. on
C. in
D. at
渗透性利尿
