在某体积可变的密闭容器里盛有适量的A和B的混合气体，在一定条件下发生反应： A+

过抛物线焦点F的直线与抛物线交于A、B两点，若A、B在抛物线准线上的射影分别为A₁、B₁，则∠A₁FB₁=______．

Nearly five years before, and with the help by our father, my sister and I planted

Some cherry tomatoes(圣女果)in our back garden. Since then for all these year we had been allowing tomatoes to self-seed where they please. As result, the plants are growing somewhere. The fruits are small in size, but juicy and taste. There are so much that we often share them with our neighbors. Although we allow tomato plants to grow in the same place year after year, but we have never had any disease or insect attack problems. We are growing wonderfully tomatoes at no cost!

保险合同除具有一般经济合同所共有特征以外，还具有的法律特征包括（）。

A.是一种要式合同

B.是一种附合合同

C.是双务有偿合同

D.是交换合同

E.是射幸合同

公司《简报》的档案保存期限应为( )。

A．短期

B．长期

C．永久

D．不定期

不符合过氧化物酶（POX）阳性率描述的是（）

A.>3%，可确定为非淋巴细胞白血病

B.<3%，可确定为淋巴细胞白血病

C.<3%，不一定为淋巴细胞白血病

D.<3%，不排除非淋巴细胞白血病

E.<3%.淋巴细胞与非淋巴细胞白血病可能同时存在