If a is a rational number, then | a | must () A. It's a positive number B. It's not a positive number C. It's a negative number D. It's not a negative number
∵ A is a rational number, a may be a positive number, a negative number and 0, | a | ≥ 0
∵ A is a rational number, a may be a positive number, a negative number and 0, | a | ≥ 0