选A。定语从句，还原语序即可知道：we cut the bread with the knife, and it is very sharp.
a.定语从句，先行词为knife，从句缺少宾语，用which.而用刀切应为cut sth with knife，with不可省略。故 用a
这题要解释太复杂、直接告诉你答案、是A
A
