Which sentence from the selection expresses the author's opinion?
F Imagine that scientists around the world have received a transmission from an alien planet telling us that spaceships were coming to Earth.
G Human beings, however, constantly threaten the balance of our planet, and not enough people are crying out for action.
H The extremely complicated interaction of temperature, atmosphere, and forces of nature on earth create a difficulty in stating simple truths.
J Scientists measuring air bubbles in ice cores dating 800,000 years old have determined that our air today contains far more C02 than ever before.