婴幼儿肺炎最主要的病理生理改变是（）
A.低氧血症
B.高碳酸血症
C.代谢性酸中毒
D.呼吸性碱中毒
E.混合性酸中毒
参考答案：A
He knows to study at his best, so he still does a good job even when he spends as much time playing
computer games as he _________. [ ]
A. is studying
B. does studying
C. has studying
D. studies
A．变压器输出端所接电压表的示数为20V
B．变压器的输出功率为200W
C．变压器输出端的交流电的频率为50Hz
D．穿过变压器铁芯的磁通量变化率的最大值为20/n2Wb/s
V
/n2Wb/s