Critical Reasoning (CR) | Lyme disease is caused by ; complete the argument

Which of the following most logically completes the argument ?

Lyme disease is caused by a bacterium transmitted to people by deer ticks. Deer ticks are almost never born infected with the bacterium, but they can pick it up from feeding on an infected host in any of the tick's three consecuctive stages of development: larva, nymph, adult. Once infected, a tick remains infected through any subsequent developmental stages, but since a tick feeds on only one host in each stage, it follows that__________________.

a.
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