13. Cold temperatures, short growing seasons, and heavy snows prevent ________ at high elevations. A. grow trees B. trees are growing C. trees growing D. trees growth 14. ________ is helping to break new ground in drug research. A. Computers are being used more B. When using computers more C. An increase in the use of the computers D. The increasing use of the computer

13. Cold temperatures, short growing seasons, and heavy snows prevent ________ at high elevations.
 A. grow trees  B. trees are growing
 C. trees growing  D. trees growth

S + prevent + something.
14. ________ is helping to break new ground in drug research.
A. Computers are being used more
B. When using computers more
C. An increase in the use of the computers
D. The increasing use of the computer

S + be + V_ing.

`\color{cyan}{\text{@Bulletproof Boy Scout}}`