Why Are Eggs Oval?

QuiZ :

All bird eggs (regardless of species) are oval and pointed at one end (instead of being, say, round, cubical or cylindrical) - do U know why?

A. because the birds wanted them to look like those of dinosaurs' so dinosaurs wouldn't destroy them
B. it was pure coincidence - just happen that all birds like their eggs that way
C. because God made it that way
D. because that shape provides the greatest chance of survival for the eggs - they cannt roll away that way. Since birds that lay eggs in other shapes had lower survival rate, they disappear over time (evolution or survival of fittest?).