There is no difficulty ________ we can't overcome in our lif

There is no difficulty ________ we can't overcome in our life.
A.which B.that C.but
With the introduction of the computer,there are no complicated problems _____can be solved.
A.that B.which
Not long after Galileo"s name,Newton invented another kind of telescope _____ mirrors instead of lenses.
A.which he used B.in which he used
She is a good dancer,______ her mother used to be.
A.as B.which
请说明选择缘由