Read and choose the correct word or phrase number (33) Rivers are one

Read and choose the correct word or phrase number (33)

Rivers are one of the world's most important natural resources. Many cities  are on large rivers, and (31)_____ every country has at least one river that plays an important part in the lives of its people. Besides transportation, rivers (32) _____  food, water for crops, water to drink, and opportunities for recreation for people who live along their banks. And in order to get water for crops, engineers sometimes build a dam (33) _____ a river and let the water become a lake behind the dam. Then people can use their water not only to irrigate fields but also to make electricity for homes and industries. However the water often becomes (34)____ when cities on river banks grow in size and the number of industries increases. We are learning that it is necessary to keep rivers clean if we want to enjoy the (35)_____ of the natural resources.

• A. A. over
• B. B. across
• C. C. among
• D. D. under