Humans have substantially altered the global nitrogen cycle over the past century. How?
A. Global warming has reduced the conversion of nitrogen gas to ammonium and other ionic forms.
B. Humans are now responsible for almost doubling the amount of nitrogen available by natural means.
C. Humans have dramatically increased nitrogen availability as a side effect of internal engine fuel combustion.
D. The number of teragrams of fixed nitrogen produced has declined in recent years.
