we are living in an age( )many things are done on computers a which b that c where d when
还有一道类似的也不太明白
we are living in an age（ which或that） we have a lot of things to learn about
第一道为什么不能用that或which.
第二道为什么不能用when
介词+关系代词（which）=where或when怎么理解