题目

--- I haven’t heard from my sister since last month.--- Don’t worry, Janet. Letters from abroad _____ be slow sometimes.A. need to                 B. can                   C. should              D. ought to 答案：B(4分)如图所示是牙膏及其盖的照片，请你仔细观察，它都涉及到了哪些物理知识?