The subordinate clause in this sentence is because I forgot to buy milk. The whole sentence is used as an adverb. Adverbs are used to answer questions such as where, when, or in this case, why.
Why did I have root beer in my cereal? Because I forgot to buy milk.
Sentence C.Our hockey team scored a sud-den
victory
Yes would you like me to explain
I took the test the answer is C.
I hate when people do this, explain what the scene is and be more specific.