This medicine millions of people's lives since it was put into use.
A:is saving
B:will save
C:has saved
D:had saved
用如图的装置研究电光效应现象,用能量为2.5eV的光子照射到光电管上时,电流表G示数不为0,移动变阻器的触点c,当电压表的示数大于或等于时0.7V,电流表示数为0,则下列说法中正确的是( )