its is correct this is correct beacuse hen you think about the question the answer will be the answer you got.
b. Do
Auxiliary verbs form the mood or tense of the other verbs in the sentence. There are three primary auxiliary verbs in the English language and these are: be, have and do.
The primary problem is the greenhouse effect. Greenhouse effect is when some gases (including CO2) absorbs some portion of sun's heat to make the atmosphere warm to support life. But when the amount of CO2 etc. increases in the atmosphere, it heats up more than it is required. So, by limiting CO2 emission, we can solve this problem.
The boys built a fort in the forest
