Choose the correct option out of the following choices.

Assertion: Network polymers are thermosetting. 

Reason: Network polymers have high molecular mass.

Assertion and Reason both are correct statement and the reason is not the correct explanation of the  assertion.

Network polymers are thermosetting because they can't be reused after heating once. As a result of substantial cross-linking caused by heating, it becomes infusible once more.
During polymerization, extensive cross-linking results in the development of a three-dimensional network that is tough, infusible, and insoluble.