Which of the following is a sign of a biological attack?

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A sign of a biological attack is the increased dead or sick wildlife. This phenomenon can occur because many biological agents affect both humans and animals. When a biological attack takes place, it can lead to widespread disease that can swiftly increase mortality rates in wildlife populations, acting as an early indicator of a potentially larger outbreak that could also impact human health. Observing unusual patterns of wildlife sickness or mortality can signal that a pathogenic organism may have been released into the environment, prompting a necessary investigation into the source.

The other options may suggest threats or incidents but are not specific indicators of a biological attack. High civilian casualties could arise from a variety of causes, including conventional warfare or natural disasters. Sudden loss of communication might indicate an electronic attack or disruption rather than a biological one. Unexpected enemy troop movements may signal tactical changes but do not provide specific evidence of biological warfare taking place.