婴儿期正确的计划免疫接种时间是()
A.脊髓灰质炎疫苗2个月
B.卡介苗2~3个月
C.麻疹疫苗4~5个月
D.牛痘6~8个月
E.乙型脑炎疫苗9~10个月
参考答案:A
______ he was at home, I would have phoned him then.
A.Had I known
B.I had known
C.Would I know
D.I would know
某时期,在一条大河的南岸的大块农田中发生某种甲虫的虫害,承包土地的农民起初在农田里喷洒某种杀虫剂R,取得较好的效果,但几年后又不得不以放养青蛙来代替喷洒农药。如图为在此时期内这种甲虫种群密度变化示意图,掘图回答下列问题:
从A点到B点,在施用杀虫剂的初期,害虫种群密度都急剧下降,但仍有极少数个体得以生存,原因是()