先心病最主要的病因是
A．遗传
B．宫内感染
C．妊母与大剂量放射线接触
D．妊娠期服用药物
E．妊母患糖尿病
参考答案：B
There ______ a park near our school since two years ago. [ ]
A. is
B. was
C. has been
D. will be
A．时刻t线圈中电流的瞬时值icosωt
B．线圈中电流的有效值I
C．线圈中电流的有效值I=
D．线圈消耗的电功率P
cosωt
