A．Cloud-to-ground lightning occurring in the U. S.
B．Types of lightning
C．Cause of lightning
D．Differences between thunder and thunderstorm
E Frequencies of thunderstorms occurring in the world and the U. S.
F Shock waves as thunder
Paragraph 5______

A. When driving trips became popular.
B. After the work to build the interstate highway system started.
C. After Internet became widely accessible.
D. After new cars were made.

A. from
B. with
C. in
D. of