C. A slow gradual change over time
A college campus including the students, birds, trees, sidewalks, and air makes up one complete ecosystem.
Ecosystem consist all the plants and animals living in an area along with their environment in other words biotic and abiotic component of the environment together consist ecosystem.
Ecosystem can be of various types- Forest ecosystem, grassland ecosystem, desert ecosystem and tundra ecosystem.
An area that has its own flora and fauna and are constantly interacting with each other can be considered as ecosystem.
Hence a college campus including the students, birds, trees, sidewalks and air can makes up an ecosystem.
To learn more about ecosystem here
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The first one, the third one, and the last one :)
1.) antigens present on the invaders.
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In my opinion the best sounding answer is C I hope this helpes<span />