Question

give reason a) decomposition reaction are opposite of combination reaction

Answer 1

In decomposition reaction one substance is decompose to give two or more than two products whereas in combination reaction two or more than two substances react to give single product

Answer 2

In a decomposition reaction a compound breaks down into two or more simpler substance whereas in a combination reaction two or more simpler substance combine together to form a new substance.

That is why decomposition reaction is opposite of the combination reaction.

Let me explain you with an example
example is on the image

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