$A$ body is moving with variable acceleration $a$ along a straight line. The average acceleration of the body in the time interval $t_1$ to $t_2$ is

• A
  $\frac{a[t_2+t_1]}{2}$
• B
  $\frac{a[t_2-t_1]}{2}$
• C
  $\frac{\int_{t_1}^{t_2} a dt}{t_2+t_1}$
• D
  $\frac{\int_{t_1}^{t_2} a dt}{t_2-t_1}$