Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.

Pollutants are clogging up the atmosphere above our planet and trapping the sun's heat. That makes our planet warmer, which causes glaciers to melt, crazy weather patterns to develop, and natural disasters spread like wildfires.

But there are other consequences that people don't always associate with climate change. One surprising fact about global warming is that it makes people more aggressive. As the world gets hotter, tempers also get fierier. A new study found that violent crimes and even war become more likely as temperatures rise. Heart rates tend to be elevated in warm weather, so people are prepped for a physical response to a situation. That’s not to say we're headed for a violent end as Earth warms up, though. Advances in everything from technology to health have sparked a decline in conflict. It's just that global warming might be slowing that decline. Climate change increases the amount of smog in the air and causes plants to produce more pollen, according to the Natural Resources Defense Council. Those combine to make breathing harder for people, which can be especially tricky for people with asthma and other respiratory illnesses.

The word “it” in paragraph 2 refers to _______.