题目

Is Oxygen the only gas ____ helps fire burn?A.it B.which C.what D.that 答案：【答案】D【解析】考查定语从句。句意：氧气是唯一能让火燃烧的气体吗？本句是限定性定语从句，从句修饰名词 the only gas，the only gas是物体，名词被the only修饰时，关系词只能用that。故选D项。平方根等于它本身的数是           。