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A. Increasing surface area to improve nutrient absorption between the digestive and the circulatory systems.

Explanation:

Our intestines, on the inside, are lined with the intestinal mucosa. This mucosa is not straightened, as it might seem macroscopically; it rather has this wavy appearance forming folds. Also, epithelial cells in this mucosal layer have lots of small finger-like extensions called the villi.

Folds and villi increase the contact surface between the nutrients in the lumen and mucosa, thus increasing the rate of absorption of these nutrients between the intestines and the blood vessels below the epithelium.

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

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