Reading, watching TV, even being online made Mary's eyelids weight a ton. Only one activity kept/could keep her senses alert: talking to John.
What's the correct option and why?
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Sign up to join this communitykept - something which was done already and it is not active now could keep - something which she should have done before and need to follow in the future
Out of these two could have
will be suitable.
"will keep" will enrich the sentence even more.