CH4和CH3(CH2)5CH3的关系是 [ ]
A．同分异构体
B．同素异形体
C．同位素
D．同系物
答案：D
His teacher worried that the student spent too much time on the computer and not enough time _______.
A．to study
B．study
C．studied
D．studying
寿命周期成本估算一般在系统开发的（）进行。
A.初期
B.中期
C.后期
D.维护期
