Which of the following best expresses the main idea of the passage

A．Marine sediments have allowed scientists to amass evidence tending to confirm that astronomical cycles drive the Earth's glacial cycles.
B．The ratio between two different isotopes of oxygen in seawater correlates closely with the size of the Earth's ice sheets.
C．Surprisingly, single-cell marine organisms provide a record of the Earth's ice ages.
D．The Earth's astronomical cycles have recently been revealed to have an unexpectedly large impact on the Earth's climat