题干

封有理想气体的导热气缸开口向下被细绳竖直悬挂,活塞下系有钩码P,整个系统处于静止状态,如图所示。若大气压恒定,系统状态变化足够缓慢。下列说法中正确的是:

A:外界温度升高,外界可能对气体做正功

B:外界温度升高,气体的压强一定增大

C:保持外界温度不变,增加钩码质量,则气体内能不变,气体一定吸热。

D:保持外界温度不变,减小钩码质量,则细绳所受拉力一定减小,气体分子单位时间作用在活塞上的分子数一定减小。

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答案(点此获取答案解析)

C

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阅读理解

    I had to knock on the taxi to get his attention. Finally, the driver, a man about 60, looked up from behind the wheel and apologized, “I'm sorry, but I was reading a letter.” He sounded as if he had a cold or a cough.

    Since I was in no hurry, I told him to finish his letter. He shook his head, explaining that he had already read it several times and almost knew it by heart. Curious, I asked whether it was from a child or maybe a grandchild. “This isn't family,” he replied. “though he might just as well have been a regular member of the family. Old Ed and I grew up together.”

    They were always friends. But since he moved away from the neighborhood 30 years ago, it'd generally just been postcards at Christmas time between them. A couple of weeks ago, Ed died. “I should have kept in touch.” He repeated this, more to himself than to me. To comfort him, I said sometimes we just didn't seem to find the time. “But we used to find the time,” he said. “Take a look.” He handed the letter over to me.

    The first sentence “I've been meaning to write for some time, but I've always delayed it.” reminded me of myself. It went on to say that he often thought about the good times they had had together. When I read the part where it said “Your friendship really means a lot to me, more than I can say because I'm not good at saying things like that”, I found myself nodding in agreement.

    We had gone several kilometers and were almost at my hotel, so I read the last paragraph: “So I thought you'd like to know that I was thinking of you.” And it was ended with “Your Old Friend, Tom.”

    “I thought your friend's name was Ed,” I said.

    “I'm Tom,” he explained. “It's a letter I wrote to Ed before I knew he'd died. I never put it in the mailbox. I guess I should have written it sooner.” His face was pale as he wiped his eyes with a handkerchief.

    When I got to my hotel room I didn't unpack right away. I had to write a letter and post it.

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