Here are the correct answers for each blank:

1. **B. as** (Light pollution is not as serious as water or air pollution.)
2. **C. occurs** (it is the type of pollution that occurs more in cities than in rural areas.)
3. **B. occur** (in cities than in rural areas.)
4. **C. produce** (used to produce the power to light the sky.)
5. **A. pollution** (Eye strain, loss of vision and stress are what people may get from light pollution.)
6. **C. but** (Too much light at night can harm our eyes but also harm the hormones that help us to see things properly.)

So the completed text would read:

Light pollution is not **as** serious as water or air pollution. Moreover, it is the type of pollution that **occurs** more in cities than in rural areas. In the past, we could sit out at night and gaze at glittering stars in the sky and light from objects in the outer space. Nowadays, cities are covered with lights from buildings, streets, advertising displays, many of which direct the lights up into the sky and into many unwanted places. The real problem is that it is very **difficult** to apply light to almost everything at night. Millions of tons of oil and coal are used to **produce** the power to light the sky. Eye strain, loss of vision, and stress are what people may get from light **pollution**. Too much light at night can harm our eyes **but** also harm the hormones that help us to see things properly.