Suppose an exchange-traded fund increases in value from $50 per share to $100 per year over a period of 4 years. During this 4-year period there are no dividends/distributions to investors and the share price already reflects deductions due to the expense ratio. What is the geometric average return per year on the fund (to the nearest percent)? A) 12.5% B) 19% C) 21% D) 25% E) 50%