A) Tropical grasses are usually C4 plants.

B) C3 and C4 plants used to live in northern Mexico and Guatemala.

C) C3 and C4 plants don’t have the same processes.

D) Tropical trees are all C3 plants.

65. Why, in the 4th paragraph, the researchers found that trends in C3 and C4 plants were different at the two locations?

A) The assumption that carbon dioxide played the major role is wrong.

B) The carbon dioxide played an important role.

C) The moisture availability was different.

D) The carbon dioxide level was different.

66. What’s the main idea of the passage?

A) Climates factors determine the plant distribution and composition of plant communities.

B) Global warming will cause major variation.

C) How has Brenner’s research team proved a truth.

D) C3 and C4 plants are important plants in determining the composition of plant communities.