SDR是指标准尺寸比，即公称内径与公称壁厚之比。
错误
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Linglingisnowreadingbooksintheschooll__________
Ithinkthe______(four)lessonisthemostdifficultinthisbook.
计算9.6÷1.92时，要先把1.92的小数点向________移动________位，原式转化为________÷192
shoes(单数)__________
Therearesomanypeopleintheshop.Youmust________yourthings.
