Nonpoint sources of pollution a. include smokestacks and automobile exhaust pipes b. are more difficult to control than point sources c. are cheaper and easier to identify than point sources d. enter ecosystems from single identifiable sources

B. are more difficult to control than point sources. This is because A. is wrong (point source), and C. is wrong, because it doesn't necessarily cost less or more to identify the sources. D. may be right, as non point sources are natural, such as snow melts or rain.

The basic repeating unit of nucleic acids are known as nucleotides. a nucleotide consists of three distinct chemical groups.