设奇函数 f (x) 在 
上具有连续导数，则()
A、
B、
C、
D、
B、
C、
D、
正确答案：
A
答案解析：
he teacher told them since light ________ (travel) faster than sound,lightning ________ (appear) to go before thunder.
For the new country to survive, ________ for its people to enjoy prosperity, new economic policies will be required.