A vertical tower stands on a horizontal plane and is surmounted by a vertical flag staff of height h. At a point on the plane, the angle of elevation of the bottom of the flag staff is a and that of the top of the flag staff is . Then the height of the tower is

• A. h tan
• B. <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>h tan </center></td><td rowspan="2"></td></tr><td align="center">tan - tan </td></table>
• C. <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>h tan </center></td><td rowspan="2"></td></tr><td align="center">tan - tan </td></table>
• D. None of these