You'd better book a ticket ______ if you want to see the 2012 London Olympic Games. [ ]

A. in advance      

B. in charge

C. in trouble  

D. in order

为了更好显示地甲状腺肿物内彩色血流信号并测速，最理想的探头频率是：

A．2MHz～2.5MHz

B．2.MHz～3MHz

C．3MHz～3.5MHz

D．5MHz～MHz

E．以上都不是