元素及其化合物的知识是化学的重点内容。下列叙述中不正确的是 A．钠呈银白色、质软

动词填空。

1. The little girl often _____ (teach) herself to do new things when she was young.

2. Driving to the centre of the city is faster than _____ (take) a bus.

3. My computer is broken, but my father doesn't agree _____ (buy) me a new one.

4. _____ (get) off the bus and we'll take the underground there.

5. Mr Wu and his students had a lot of fun _____ (travel) to South Hill.

6. Look, the students of Class One _____ (plan) to go on a school trip next week.

如图所示，重为500N的明明站在水平地面上，用力拉着跨过定滑轮的竖直细绳，此细绳悬挂着重为200N的水桶保持静止状态．不计定滑轮的摩擦，则明明所受的合外力是（　　）

A．0

B．200N

C．300N

D．700N

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临床资料：男，49岁，腰部不适半年余。

超声综合描述：右肾失常态，下极可见6．7cm×6．5cm中等回声（图1），边界尚清晰，内回声欠均匀，向外突起，CDFI：内部及周边可见丰富动、静脉血流信号。见彩图。

右肾静脉、下腔静脉肝后段管腔增宽，内可见中等回声（图2、图3），CDFI：该段内血流信号充盈缺损。

超声提示（）

A.右肾实性占位，考虑肾癌

B.右肾静脉实性回声，考虑癌栓

C.下腔静脉实性回声，考虑癌栓

D.以上各项均是

为什么在浇铸汽轮机轴瓦时，瓦胎要先预热到一定范围的温度？