The type of sentence is a: A. Complex Sentence.
A complex sentence refers to the type of sentence that consists of a dependent clause and an independent clause. Such dependent clause and independent clause are usually separated by a comma.
We can see here that "Since she forgot the key to the lock," is a dependent clause and "Andrea could not open the mailbox." is an independent clause. Thus it is a complex sentence.
Learn more about complex sentence on https://brainly.com/question/17538242
