Question

how do nicotine and tobacco cause cancer?
by decreasing blood - sugar levels
by damaging or changing a cells dna
by increasing heart rate
by increasing phlegm and mucous production

Explanation:

Cancer is caused by changes (mutations) in the DNA of cells. Nicotine and tobacco contain harmful substances that can damage or change a cell's DNA. Decreasing blood - sugar levels, increasing heart rate, and increasing phlegm and mucus production are not the direct mechanisms by which nicotine and tobacco cause cancer.

Answer:

by damaging or changing a cell's DNA