The Pollen, or Sperm, from a Red Maple Tree Is

The pollen, or sperm, from a red maple tree is unable to fertilize the eggs from a sugar maple tree. This is an example of what type of isolating mechanism?

A) mechanical isolation
B) habitat isolation
C) temporal isolation
D) hybrid sterility
E) gamete isolation This is an example of gamete isolation because the gametes from the two species are unable to fuse.