Thenextthing____mustbedoneistomakeaplan.这个定语从句中必须用that么which可以么先行词是thing必须用that么?先行词被thenext修饰了,必须用that么?

　　Thenextthing____mustbedoneistomakeaplan.这个定语从句中必须用that么which可以么

　　先行词是thing必须用that么?

　　先行词被thenext修饰了,必须用that么?