in my opinion life in the 21st century is much easier than（）In my opinion,life in the twenty-first century is much easier than ________.A.that used to be B.it is used to C.it was used to D.it used to be 为什么不选A,that不也可指代么?我学比较级的时候用来代指天气等不可数名次都用that啊 怎么这个不是呢》?

in my opinion life in the 21st century is much easier than（）
In my opinion,life in the twenty-first century is much easier than ________.
A.that used to be B.it is used to C.it was used to D.it used to be
为什么不选A,that不也可指代么?我学比较级的时候用来代指天气等不可数名次都用that啊 怎么这个不是呢》?