Improving Clarity in Sentence Structure

During the lecture on environmental policy, Professor Johnson emphasized that reducing carbon emissions is not only essential for combating climate change but, it is also crucial for improving public health. Students frequently responded to this statement, expressing their enthusiasm for sustainability efforts on campus. However, many students were unsure whether to employ formal or informal language in their discussions.

Which of the following sentences best replaces the underlined portion to improve the clarity and correctness of the passage?