The correct options are
A Global warming mainly results from an increase in the levels of greenhouse gases such as carbon dioxide and methane in the atmosphere.
C Global warming reduces pH levels in the oceans.
Global warming mainly results from an increase in the levels of greenhouse gases such as carbon dioxide and methane in the atmosphere. Carbon dioxide dissolves in water to form carbonic acid. This would result in an apparent decrease in pH levels in the oceans, adversely affecting marine life.