Science 10 points

the greenhouse effect traps heat near earth's surface. greenhouse gases in the atmosphere—carbon dioxide, water vapor, methane, and nitrous oxide—allow the shorter waves of ultraviolet radiation from the sun to enter earth's lower atmosphere. however, these gases do not allow the longer wavelengths of infrared radiation to pass back out to the outer atmosphere. the result is that the lower atmosphere heats up. this result happens naturally, but it can be accelerated by human activities. since the late 1800s, human activity has greatly altered the amount of greenhouse gases that enter the atmosphere. burning fossil fuels, farming, and clearing forests, as well as industry, have all contributed to increases of carbon dioxide (co2), methane, nitrous oxide, and other greenhouse gases in the atmosphere. the result is that temperatures in the troposphere have gone up. many scientists identify this increase in greenhouse gas emissions as contributing directly to global warming. this suggests that human activity has had an impact on the global climate.
the author argues that in modern times, the greenhouse effect *
a- keep the earth warm
b-has increased because of human activity
c-is in a natural cycle of changing
d-has changed local weather patterns