Fill in the blank with the appropriate Modal from the options given below.

'If you smell gas, you _________ phone the emergency number.'

must

'If you smell gas, you must phone the emergency number.'

• The sentence describes a situation where smelling gas requires immediate action due to the potential danger.
• "Must" expresses strong obligation or necessity, emphasizing the crucial importance of phoning the emergency number in such a situation.
• The other options are less suitable:
  • "Can" simply indicates ability, not the urgency of taking action.
  • "May" suggests permission or possibility, not the necessary course of action.
  • "Should not" implies avoidance, which is not the intended message in this scenario.