The doctor did what he could ---- the dying man .

The doctor did what he could ____ the dying man
.A.save B.to save C.saved D.saving
为什么选B,请详解,
因为
The doctor did what he could to save the dying man =The doctor did what he could do to save the dying man
因为句中有do,为了避免重复,所以,省略了do.