有关高一英语定语从句的三道选择题

有关高一英语定语从句的三道选择题
1.The knife [ ]we used to cut the bread is very sharp
A.which B.with that C.with it D.with which
2.You may take anything useful [ ]
A.which you want B.you want them C.what you want D.you want
3.Is oxygen the only gas[ ]helps fire burn?
A.it B.which C./ D.that