虱传回归热病后获得性免疫力为（）。
A.终生免疫力
B.交叉免疫力
C.持久免疫力
D.无免疫力
E.阶段免疫力
参考答案：E
---- Why have you bought so much food ?
----My good friends ________ to stay with me for the weekend.
A．are coming
B．have come
C．come
D．will have come
提高性能的物质（）
