Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Carbohydrates, which are sugars, are an essential part of a healthy diet. They provide the main source of energy for the body, and they also function to flavor and sweeten foods. Carbohydrates range from simple sugars like glucose to complex sugars such as amylose and amylopectin. Nutritionists estimate that carbohydrates should make up about one-fourth to one-fifth of a person's diet. This translates to about 75-100 grams of carbohydrates per day.

A diet that is deficient in carbohydrates can have an adverse effect on a person's health. When the body lacks a sufficient amount of carbohydrates it must then use its protein supplies   for energy, a process called gluconeogenesis. This, however, results in a lack of necessary protein, and   further health difficulties may occur. A lack of carbohydrates can also lead to ketosis, a build-up of ketones in the body that causes fatigue, lethargy, and bad breath.

What does the word "this" refer to?