Use the Trigonometric Substitution U = a Sin θ,Where - $\sqrt { a ^ { 2 } - u ^ { 2 } }$

Multiple Choice

Use the trigonometric substitution u = a sin θ,where - π/2 < θ < π/2 and a > 0 to simplify the expression $\sqrt { a ^ { 2 } - u ^ { 2 } }$ . ​

A) ​a csc θ
B) ​a sin θ
C) ​a tan θ
D) ​a cos θ
E) ​- a cos θ