What does this in paragraph 6 refer to?

a. In conclusion, we can improve the environment in our school in many ways and even small actions can make a big difference.

b. There are two things we can do to improve the environment in our school.

c. First, we should clean up all areas regularly.

d. Second, let's plant more trees in the school yard.

e. For example, we can organize clean-up activities at weekends.

f. This helps reduce CO2 and make the air clean.

Mark the letter A, B, C, or D to indicate the correct arrangement of the sentences to make a meaningful paragraph/letter in each of the following questions.

Mark the letter A, B, C or D to indicate the word whose underlined part differs from that of the other three in pronunciation in each of the following questions.

Mark the letter A, B, C or D to indicate the word whose underlined part differs from that of the other three in pronunciation in each of the following questions.