在碱性溶液中，被铁氰化钾氧化，加正丁醇萃取，醇层显蓝色荧光的是A．维生素A B．维生

To learn English well, we should find opportunities to hear English ______ as much as we can. [ ]

A. speak

B. speaking

C. spoken

D. to speak

海拔5895米的乞力马扎罗山是非洲第一高峰，山顶冰峰峭立，白雪皑皑，形成奇异的“赤道雪峰”景观．为什么会这样呢？算一算你就会明白了．该山上海拔900米处气温为20℃，山顶的气温会是多少呢？（　　）

A．约20℃

B．约-5℃

C．约-10℃

D．约-20℃

Great changes _____ in my hometown since 1980.     [ ]

A. have been taken place              

B. took place    

C. have taken place              

D. were taken place

倒扣器在下钻过程中，切忌转动工具。

ADPCM编码后的信号通常（）。

A.是模拟信号

B.是数字信号

C.是数字信号和模拟信号都有可能

D.既不是数字信号也不是模拟信号