Why Is It Important That Gametes Be Haploid

Why is it important that gametes be haploid?

A) Because they need to have extra chromosomes to develop into a complete organism.
B) Because the sperm and egg cells need to be very small.
C) It isn't really important. That's just how it works.
D) Because if they weren't, when they joined at fertilization the resulting zygote would have twice the normal number of chromosomes.
E) Because if they weren't, they could not properly join at fertilization and there could be no reproduction.