If a balloon lying at an altitude of 30 m from an observer at a particular instant is moving horizontally at the rate of $1 \mathrm{~m} / \mathrm{s}$ away from him, then the rate at which the balloon is moving away directly from the observer at the $40^{\text {th }}$ second is (in $\mathrm{m} / \mathrm{s}$ )

• A

  $1.2$
• B

  $0.9$
• C

  $0.6$
• Correct answer
  D

  $0.8$

Hence, the correct answer is option: 'D'.