题目

已知：如图，E、F是□ABCD对角线AC上两点，AF=CE. 求证：BE∥DF． 答案：证明：∵□ABCD ∴AB∥DC,  AB=CD     ∴∠BAE=∠DCF     在△ABE和△CDF中 ∵ ∴△ABE ≌ △CDF       ∴BE=DF           All of them didn't stop fighting against the fire ________ it was out. [　　] A．afterB．until C．unlessD．when