I’ve been trying to phone Charles all evening, but there must be something wrong with the network; I can’t seem to_____.

A．get through

B．get off

C．get in

D．get along

患儿，女，7岁，乳牙龋坏较多，已充填。六龄牙已萌齐，窝沟深，部分窝沟有着色，无明显龋坏，要求预防。

该患者首选的龋病预防措施是窝沟封闭，应选用何种浓度的磷酸进行酸蚀

A．10%～20%

B．20%～30%

C．30%～40%

D．40%～50%

E．50%～60%