Species a Is Divided by a Newly Formed River to Become

Species A is divided by a newly formed river to become two populations.These populations diverge to become two different species,B and C.If the river is dammed upstream,eliminating an isolating mechanism between B and C,what will happen?

A) Gene flow will resume.
B) Species B and C will be considered subspecies.
C) Species B and C will remain genetically isolated.
D) Species will unite to form a hybrid species, D.