20. Dry stone walls, when properly __________, will last several lifetimes.

　 A. is built　　 B. building　　 C. to build　 D. built

19. A recent research indicates that __________ when they are greatly encouraged by their parents, will the kids improve their ability of dealing with problems.

A.　　 only　 　B. just　 C. Still　 D. yet

18. With so many life’s challenges __________, the injured soldier was lucky to have a trained dog around to provide support.

A. overcame　　　　B. overcoming　　　C. to overcome　　　D. being overcome

17. _____________ the ability to speak, so he had to communicate with his friends and family by using sign language.

　 A. As he lost　　 B. He lost　　 C. Having lost　　　 D. His being lost