Fill in the blanks with the correct words: A dam is a ___ wall built a river to hold back water. Behind the dam, the water forms a small . Dams provide water drinking, farming ___ factories.

Here are the completed sentences:

• 16. A dam is a large wall built...

  • Dams are typically substantial in size, making "large" the most appropriate descriptor for the wall.
  • The correct option is D. large.

• 17. ...wall built across a river to hold back water.

  • Dams are constructed from one bank of a river to the other to block the flow of water.
  • The correct option is C. across.

• 18. Behind the dam, the water forms a small lake.

  • The body of water impounded by a dam is commonly referred to as a reservoir, which is a type of artificial lake.
  • The correct option is A. lake.

• 19. Dams provide water for drinking, farming...

  • The preposition "for" indicates the purpose or use of the water provided by dams.
  • The correct option is B. for.

• 20. ...drinking, farming and factories.

  • "And" is used to connect the last item in a list of purposes for which dams provide water.
  • The correct option is C. and.

Here are the answers: 16. $\boxed{\text{D. large}}$ 17. $\boxed{\text{C. across}}$ 18. $\boxed{\text{A. lake}}$ 19. $\boxed{\text{B. for}}$ 20. $\boxed{\text{C. and}}$