6、Many kids turn up their noses at the thought of eating fish because, well, it can smell "fishy".  While it is usually a healthy source of protein in the diet, there may be several reasons to avoid biting into a forkful of fish. Before you break out the tartar sauce, learn some facts about fish to keep yourself and the environment healthy.

Some fish may contain harmful chemicals, which can be the result of both natural causes and water pollution. Mercury is a poisonous chemical, which occurs naturally in oceans and the Earth’s crust, but also comes from man-made sources, such as pesticides, burning garbage, and the releasing of fossil fuels.

Polychlorinated biphenyls (PCBs), a group of chemicals, can also be found in some fish. These dangerous man-made chemicals were used in many industries until 1977, when they were banned.  PCBs were released or leaked into the air and water and have been transported around the globe.

How do mercury and PCBs end up in fish and the food supply? When they are released into the air, they attach themselves to particles. These particles settle on the ground and in the water and are eventually eaten by microscopic organisms. Small fish eat the micro-organisms, and large fish eat the small fish and on up the food chain.

Because they can negatively affect your health, the U.S. Environmental Protection Agency advises that kids younger than 15 years old avoid fish that contain high levels of mercury and PCBs. These include shark, swordfish, king mackerel, and tilefish.

There are also many types of fish that are good for you, but because of overfishing are in danger of being wiped out. Some are being fished in the wild so much that they cannot reproduce fast enough to survive. Others are being farmed in ways that are not environmentally friendly. These fish include red snapper, Atlantic salmon, blue-fin tuna, and king crab.

Despite these problems, there are several fish that are both healthy and sustainable, such as Alaskan salmon, American catfish, Pacific cod, and several farm-raised fish and shellfish.

51. How do mercury and PCBs come to our food supply?

A. Mercury and PCBs--water --fish -- particles --food supply

B. Mercury and PCBs--particles -- organisms-- fish -- food supply

C. Mercury and PCBs--particles -- fish -- organisms -- food supply

D. Mercury and PCBs―organisms--ground and water -- fish -- food supply

52. The underlined word "they" (paragraph 5) refers to "      ".

A. fish       B. particles     C. microscopic organisms      D. mercury and PCBs

53. According to the passage which group of fish can be safely eaten by young kids?

A. Shellfish; tilefish; Alaskan salmon.

B. King crab; king mackerel; Pacific cod.

C. Pacific cod; Alaskan salmon; American catfish.

D. Pacific cod; Atlantic salmon; swordfish.

54. The writer is most likely to agree that         .

A. we should be careful when deciding what kind of fish to eat

B. overfishing is the main cause of fish poisoning

C. most types of fish can be safely raised on fish farms

D. the government should ban children under 15 from eating fish

55. It can be inferred from the passage that          .

A. children like eating fish because it smells fishy

B. if you want to be more environmentally friendly, eat less fish

C. PCBs were not allowed to be used in food industry before 1977

D. adults might be less affected by high chemical levels in fish