Consider the following discussion that occurs in a marine biology class.
(professor) A few years ago, our local government passed a number of strict environmental laws. As a result, Sunrise Beach looks nothing like it did ten years ago. The water is cleaner, and there's been a tremendous increase in all kinds of marine life--which is why we're going there on Thursday.
(woman) I don't know if I agree that the water quality has improved. I mean, I was out there last weekend, and it looked all brown. It didn't seem too clean to me.
(professor) Actually, the color of the water doesn't always indicate whether it's polluted. the brown color you mentioned might be a result of pollution, or it can mean a kind of brown algae is growing there. It's called "devil's apron," and it actually serves as food for whales.
(man) So when does the water look blue?
(professor) Well, water that's completely unpolluted is actually colorless. But if often looks bluish-green because the sunlight can penetrate deep down, and that's the color that's reflected.
(woman) But sometimes it looks really green. What's that about?
(professor) OK, well, it's the same principle as with "devil's apron": the water might look green because of different types of green algae there--gulfweed, phytoplankton. You all should finish reading about algae and plankton before we go. In fact, those are the types of living things I'm going to ask you to be looking for when we're there.
Referring to the discussion in the marine biology class, what can make ocean water look brown, according to the professor?
a. pollution and optical illusions
b. pollution and sharks
c. algae and pollution
d. cloudy skies and sand
e. sand and optical illusions
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