Meaning of I must fly

[Rich7]

Test No. incompl/inter-87 "Present Simple (4)", question 2

Suddenly Sally looked at her watch: 'I must .......... My train leaves in ten minutes.'

(a) fly
(b) flow
(c) flee
(d) flea

Test No. incompl/inter-87 "Present Simple (4)", answer 2

Suddenly Sally looked at her watch: 'I must fly. My train leaves in ten minutes.'

Correct answer: (a) fly

Your answer was: incorrect
Suddenly Sally looked at her watch: 'I must flee. My train leaves in ten minutes.'
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what about flee here?

flee: run away quickly

[Alan]

I must fly means I must go quickly because I'm late. Flee means run away from danger as in The villagers had to flee when they saw the giant wave coming towards them.
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English as a Second Language
You can read my ESL story Progressive Forms

[Rich7]

Good point there.