What is the main difference between PCI-X and PCI-E?

What is the main difference between PCI-X and PCI-E?

A.
PCI-X is serial and PCI-E is parallel

B.
PCI-E is Serial and PCI-X is parallel

C.
PCI-X is 32-bit and PCI-E is 64-bit

D.
PCI-E is 32-bit and PCI-X is 64-bit