If the angles of elevation of a tower from two points distance a and b (a > b) from its foot and in the same straight line from it are 30° and 60°, then the height of the tower is?

• A. $\sqrt {a + b} $$
• B. $\sqrt {ab} $$
• C. $\sqrt {a - b} $$
• D. $\sqrt {\frac{a}{b}} $$