题目

Running on the beach can certainly be a good thing. It's completely different from running on the pavement，a track or trails with soft surfaces.For starters，sand can be a challenge because it has an uneven（不平的）surface and constantly moves under your feet. As you push off，you're going to lose some of your push as the sand moves. So，you're not going to be able to push yourself forward as you would on a track or pavement！But this unevenness has an upside: It gives your body an extra workout，forcing you to exercise muscles that don't get as much use during runs on firm surfac3．如图所示是通过电阻A、B的电流与其两端电压的关系图象，分析图象可知RA＜RB（填“＞”、“=”或“＜”）．若将电阻A和B串联后接在电压为6V的电源两端，则电阻A和B消耗的电功率之比PA：PB=1：2．