右心衰竭时呼吸困难的机制，主要为（）
A.迷走神经兴奋性增高
B.心肌供血减少
C.右心房及上腔静脉压力升高
D.下肢静脉回流增多
E.肺淤血
参考答案：C
--- Mum, where is my toy cowboy? I________it under the bed.
--- It_____inside your toy bucket. Remember, that’s where all your toys are supposed to go.
A．have put; is
B．put; is
C．have put; was
D．put; was
在实验室里电解1.8g水，问要标准状况下可制得氢气多少升？（标准状况下氢气的密度为0.0899g/L）
