已知空气的主要成分是氮气和氧气，某课外活动小组设计测定空气中氧气含量的实验，实验装置如图所示： （1）该实验中红磷稍过量，目的是
Look at the boys. They _____ games at school. A. play B. playing C. are play D. are playing
Americans are buying more products and services than ever before through the Internet. And experts say the popularity of online sales is likely to spread to other countries. Online sales now repres
Large amounts of grass ______ to prevent wind or water ______ the soil. A. has been planted; from carrying away B. have been planted; carrying away C. have planted; being carrying awa
鸟类排便频繁的原因是（ ） A．直肠短，不贮存粪便 B．小肠短，不贮存粪便 C．消化不完全
(18分) (1)甲同学要把一个量程为200μA的直流电流计，改装成量度范围是0~4V的直流电压表。 ①她按图1所示电路、用半偏法测定电流计的
地方势力威胁中央政权的危险基本解除始于 A．隋唐时期 B．北宋时期 C．元朝时期 D．清朝时期
