In developing the main point, the author makes use of ______.

A．streams that do not flow directly to open bodies of water
B．cesspools and septic tanks that contaminate water supplies
C．storm and waste disposal sewers that have been combined
D．the undesirable odors of sewage

A．prove that the city refused to deal with pollution
B．prove that medical science once knew little about pollution
C．introduce the idea of contaminated water supplies
D．recall a historical fact