Bound to come

[lira]

Test No. incompl/elem-32 "Responses (5)", question 6

Jill: 'I do hope they make it tonight - it's very late you know.'
Frank: '.........'

(a) They're tied to come.
(b) They're destined to come.
(c) They're forced to come.
(d) They're bound to come.

Test No. incompl/elem-32 "Responses (5)", answer 6

Jill: 'I do hope they make it tonight - it's very late you know.'
Frank: 'They're bound to come.'

Correct answer: (d) They're bound to come.

Your answer was: incorrect
Jill: 'I do hope they make it tonight - it's very late you know.'
Frank: 'They're tied to come.'
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Please explain whats mean 'They're bound to come.'

Thanks

[Alan]

This means it is certain they will come.
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English as a Second Language
You can read my ESL story Present Simple