A balloon of weight w is falling vertically downward with a constant acceleration a (< g). The magnitude
of the air resistance is

• A

  $\omega$

• B

  $w\left(1+\frac{a}{g}\right)$

• C

  $w\left(1-\frac{a}{g}\right)$

• D

  $w\frac{a}{g}$