Assertion : Soldiers are asked to break steps while crossing the bridge. Reason : The frequency of thick marching may be equal to the natural frequency of bridge and may lead to resonance which can break the bridge.

• Correct answer
  A

  If both $A$ and and Reason are true and Reason is the correct explanation of Assertion.
• B

  If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
• C

  If Assertion is true but Reason is false.
• D

  If Assertion is false but Reason is true.

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