两题考查得都是主谓一致原则.
1. B
此题需要填入谓语动词,而CD都是非谓语形式,排除；
two thirds of carbon dioxide做句子主语时,动词单复数选择采用就近原则,所以是和carbon dioxide（二氧化碳）保持一致,所以用单数has.
2. D
1) 该题后句是个强调句型：
it is not learning about english ,but learning english that ___important for learners of english as a foreign language.
强调的是主语not learning about english, but learning english,
2) 把句子还原成正常句序：
not learning about english but learning english ____ important for learners of english as a foreign language.
3) not...but...连接并列主语时,谓语动词的单复数形式也采用就近原则,即和learning english保持一致,所以用单数形式.