Why are the 1300 nm and 1550 nm regions of the fiber spectrum primarily used for optical communications？（）

A、A. They are both regions of lowest loss.

B、B. They are both regions of low reflections.

C、C. They are both regions of low index of refraction.

D、D. They are both regions of low chromatic dispersion.

E、E. They are both regions of low Polarization Mode Dispersion （PMD）.