Meaning of know

[Helen]

English Language Tests, Intermediate level

ESL/EFL Test #101 "Phrasal Verb: Walk", question 10

knew we shouldn't have trusted them because they suddenly left and walked ......... on us.

(a) in
(b) out
(c) above
(d) to

English Language Tests, Intermediate level

ESL/EFL Test #101 "Phrasal Verb: Walk", answer 10

knew we shouldn't have trusted them because they suddenly left and walked out on us.

Correct answer: (b) out

Your answer was: correct
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Hi teacher,
Would you please tell me what is the meaning of "know" in the beginning of the sentence?
Thank you.

[Mister Micawber]

.
It appears to me that the first word(s) of the sentence have been inadvertantly deleted. It should read:

I knew we shouldn't have trusted them...
.
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Native English teacher at Mister Micawber's