Why Was the "Ground Zero" of the First Nuclear Explosion

Why was the "ground zero" of the first nuclear explosion in July 1945 near Los Alamos, New Mexico, covered with glass?

A) Larger than expected, the explosion destroyed a town with houses that still had many glass windows.
B) The first nuclear bomb was embedded in one ton of glass to allow special cameras to record the nuclear meltdown.
C) The detonation had shattered the glass of nearby homes and vehicles and sucked the debris to the center.
D) The heat of the explosion melted the sandy surface of the crater and turned it into glass.