题目

两组邻边分别相等的四边形叫做“筝形”.如图,四边形ABCD是一个筝形,其中AD=CD,AB=CB,詹姆斯在探究筝形的性质时,得到如下结论: ①AC⊥BD;②AO=CO=AC;③△ABD≌△CBD. 其中正确的结论有 A.0个            B.1个 C.2个            D.3个 答案：D There ________ so many cars and pollution in Beijing．That's a problem． [　　] A． have B． has C． is D． are