质量为2kg的物体受到大小分别为15N和10N的两个力作用，则该物体的加速度大小可能是（　　）

如图所示，将凸透镜看作是眼睛的晶状体，光屏看作是眼睛的视网膜，烛焰看作是眼睛观察的物体。拿一个近视眼镜给“眼睛”戴上，光屏上出现烛焰清晰的像，而拿走近视眼镜则烛焰的像变得模糊，那么在拿走近视眼镜后，下列操作能使光屏上重新得到清晰像的是（）

某同学在做“比较不同物质吸热能力”的实验时，使用相同的电加热器给水和煤油加热，他得到如下数据：

    Mr. Zhang and Mr. Wang work in the same office. They're good friends.Mr. Zhang says to Mr. Wang in the office,“I would like to have a small party in my home on Sunday evening. Would you and your wife like to come?”

    Mr. Wang says,“Thanks a lot. I'd like to. Let me tell my wife about it.”So Mr. Wang goes to the other office and calls his wife. About half an hour later, he comes back and looks very worried.

    “What's up?” asks Mr. Zhang,“Is your wife at home?”

    “No,” says Mr. Wang.“She isn't at home now , but my little daughter is in .My little daughter says her mother goes out to look for her.”