题目

—I’d like to invite you to a concert this evening. —Thank you,but I’ll be free I’m not sure at that moment. A.while B.if C.when D.whether 答案：D解析: 本句的正常语序为“but I’m not sure whether I’ll be free at that moment.”意为“我不能确定到时是否有空”。whether引导宾语从句，跟在作表语的形容词后作宾语。解方程 x2－2x－8＝0