题目

Healthy knees aren’t the main consideration in choosing high heels, but new research says chunky heels are just as bad for the knees as spindly stilettos（细高跟鞋）. “It takes a long time to feel the effects of knee osteoarthritis（骨关节炎）—and once you do it, it is too late,” said Dr. Casey Kerrigan, leading researcher of the study and associate professor at Harvard Medical School’s department of physical medicine. “I compare it to smoking—one cigarette is not painful, but over a lifetime it is. Wide-heeled shoes feel comfortable, so women wear them all day long,如图，一理想变压器原副线圈匝数之比为4：1，原线圈两端接入一正弦交流电源；副线圈电路中R为负载电阻，交流电压表和交流电流表都是理想电表．下列结论正确的是（ ）A．若电压表读数为6V，则输入电压的最大值为VB．若输入电压不变，副线圈匝数增加到原来的2倍，则电流表的读数减小到原来的一半C．若输入电压不变，负载电阻的阻值增加到原来的2倍，则输入功率也增加到原来的2倍D．若保持负载电阻的阻值不变．输入电压增加到原来的2倍，则输出功率增加到原来的4倍