Though Mr Shi got a lot on his mind, he ____ (答复)to the girl’s e-mail as soon as he received it.
—Alice, why didn’t you come yesterday?
—I ______, but I had an unexpected visitor.
甲、乙两小车同时、同地沿直线开始运动,它们的s﹣t图象分别如图(a)、(b)所示.以下说法中正确的是
①摩擦生热,②热水袋取暖,③钻木取火.