Answer:
A). As the United States grew into a fast-paced society,
Step-by-step explanation:
The given sentence exemplifies a complex sentence as it consists of a dependent or subordinate clause followed an independent clause. The dependent clause is characterized as the clause that cannot stand on its own and is dependent on the principal or independent clause to accomplish its meaning. Such clauses begin with a subordinating conjunction. In the given sentence, the clause 'As...society,' is the dependent clause as it justifies the above definition(begins with subordinating conjunction 'as' and its dependency on main clause to complete its meaning). Therefore, option A is the correct answer.