题目

The president promised to keep all the board members _________ of how the negotiations were going on. A. inform          B. informing        C. be informed        D. informed    答案：D 解析: 略作用于一个点上的三个力大小分别为F1=3N，F2=5N，F3=7N，它们的合力大小可能为A.0B.2NC.15ND.18N