It's some change in the conditions around the organism, which the organism can detect and respond to. A change in temperature, more light, or something physically poking the organism are examples.
The answer is A. a light wave
C and D both need water as a medium and sound needs air to vibrate in order to travel. light, on the other hand, does not need a medium
a social group can be defined as two or more people who interact with one another, share similar characteristics, and collectively have a sense of unity. Other theorists disagree however, and are wary of definitions which stress the importance of interdependence or objective similarity. Instead, researchers within the social identity tradition generally define it as "a group is defined in terms of those who identify themselves as members of the group."[3] Regardless, social groups come in a myriad of sizes and varieties. For example, a society can be viewed as a large social group.
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