A ladder 15 m long just reaches the top of a vertical wall. If the ladder makes an angle of 60° with the wall, then the height of the wall is

• A. $15\sqrt 3 \,m$$
• B. $\frac{{15\sqrt 3 }}{2}\,m$$
• C. $\frac{{15}}{2}\,m$$
• D. $15\,m$$