121. As a result of medical advances, many people that might at one time have died as children of such infections as diphtheria, pneumonia, or rheumatic fever now live well into old age.
(A) that might at one time have died as children
(B) who might once have died in childhood
(C) that as children might once have died
(D) who in childhood might have at one time died
(E) who, when they were children, might at one time have died

OA is B

Can someone explain in detail ?