I think----it will clear up this afternoon and ---they will

I think____it will clear up this afternoon and ___they will come to say good-bye to us.
A.that;/ B.that;when
C./;that D.when
话说标准答案我是知道过的,主要是求详解
第一个空为何要省略掉that,第一个都省掉了第二个为啥不省掉