Question

give one property because of which viruses are regarded as being on the borderline of living and nonliving

Answer 1

Unless and until they enter a host body, they do not do anything and remain like dead but as they enter the host body or patient, they become active like living organisms.
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Answer 2

when virus are in outside of living body then they act as nonliving but when they enter in a living body then they function as living body (like, they start to reproduce etc ). That's why virus are regarded as being on the borderline of living and nonliving .