How are catastrophism and gradualism different

Catastrophism is associated with major changes that occurs once at a time in a species while gradualism is associated with tiny changes that occurs over a period of time that finally contributes a bigger evolutionary change.

Explanation:

The events that are sudden, short-lived, violent occurring worldwide thereby changing the shape of the Earth is Catastrophic. This process is opposite to contrast to  gradualism which is also called as uniformitarianism. This process is associated with slow and also incremental changes. Erosion is the best example for this process.

The formation of major mountain chains and great floods are some of the examples of Catastrophism. Both are associated with the changes that occurs in a species by the time taken for the changes determines whether it is Catastrophism or gradualism.

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the answer to your question is:

c. complementary base-pairing