Hi, the author starts by telling us that people generally only look at the biggest and most obvious issues. They then tell us that ozone is a clear issue, but water pollution is more obvious, so the public won't pay attention to the ozone issue.
The first sentence is a premise that supports the overall conclusion, which is what B says.
What it could potentially also explain why people are paying attention to water, that is not how the author uses the first sentence. The author never actually explains why he public is so aware of the severity of the water problem.
