He began to __________ because every means _______ tried already.
A:lose heart; has been
B:lose heart; had been
C:lose his heart; was
D:lose his heart; had
下列哪一项不是小肠结构与吸收功能相适应的特点( )
古人诗有“风定花犹落”一句,素来无人能对,王安石借用王籍原诗“鸟鸣山更幽”来对。这样对仗体现了( )
①联系的观点 ②静止和运动是统一的
③静止和运动是对立的 ④实践是认识的基础
在测量一节干电池的电动势和内阻实验中,小明设计了如图甲所示的实验电路
若某家庭使用功率为2000W,容积为100L的电热水器,加满水后,把初温是10℃的水加热到70℃,这些水需要吸收的热量是 ____ J,若该电热水器的效率为75%,则需要加热的时间为____ s.c水=4.2×103J/(kg•℃).