" Yes,-----," the girl said."I would be happy to help you."

" Yes,_____," the girl said."I would be happy to help you."
A off course B course
C because D of course
请问这道英语单选选哪个呢?
请告知为什么选那项,别的为什么不选?最后还请翻译一下这句话..
还请告知would后面为什么用be这个原形呢?时不时因为would是情态动词呢?