慢性根尖周囊肿的诊断主要依靠（）。
A.牙髓电活力检查
B.温度试验
C.咬诊和叩诊
D.探诊和扪诊
E.X线检查
参考答案：E
My friend, who _____ on the International Olympic committee all his life, is retiring next month. [ ]
A. served
B. is serving
C. had served
D. has served
氢气系统必须定时放水的原因是什么？
