Fact 1: Thyroid hormones increase the rate of metabolism in cells throughout the body. Fact 2: Thyroid hormone is made by the thyroid gland. Given the two facts above, what increases metabolism in cells throughout the body?

Options are: 1). lymph nodes 2). aerobic capacity 3). puberty 4). thyroid gland 5). Smoking 6). smoking cigarettes 7). cells 8). heat energy
Answer:
4).