Final answer:
The sentence that demonstrates proper subject/verb agreement is 'Each book is too long.'
Step-by-step explanation:
The sentence that demonstrates proper subject/verb agreement is:
Each book is too long.
In this sentence, the singular pronoun 'each' is followed by the singular verb 'is', which agrees in number.
When using 'each', it is important to remember that it is always followed by a singular verb.
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