原发性血小板增多一般不见于 （）
A.慢性粒细胞白血病
B.急性感染
C.真性红细胞增多症
D.骨髓纤维化
E.原发性血小板增多症
参考答案：B
如图所示，相距为0.2m的平行金属板A、B上加电压U=40V，在两板正中沿水平方向射入一带负电小球，经0.2s小球到达B板，若要小球始终沿水平方向运动而不发生偏转，A、B两板间的距离应调节为多少？（g取10m/s2）
用所给单词的适当形式填空
1. There are _______ (million) of birds in the forest.
2. You _______ (fail) the exam unless you work hard.
3. The cake _______ (make) by my mother yesterday.
4. The abacus was invented in the _______ (six) century.
5. I _______ (know) nothing about it until you told me.
