Final answer:
The complex sentences are those with an independent clause and a subordinate clause, which in this case are options 2 and 4 from the list, making the answer B.
Step-by-step explanation:
To identify complex sentences from the provided options, one must recognize that a complex sentence consists of a main clause and one or more subordinate clauses. An independent clause can stand alone as a sentence, while a subordinate clause (also known as a dependent clause) cannot stand alone and is connected to the independent clause using a conjunction or relative pronoun. Two sentences from the options contain subordinate clauses: “When the rider pedals, the cranks turn a sprocket” and “When the wind blows, it spins the blades”. Each of these sentences has a main clause and begins with a subordinate clause introduced by 'when', making them complex sentences.
Hence, the correct answer to the question is option B, which includes sentences 2 (“Birds may be hurt by the spinning blades”) and 4 (“When the rider pedals, the cranks turn a sprocket”). There is a common misconception that option C might be correct since it mentions the environment, which is highly relevant to the subject matter, but this option doesn't provide sentences that follow the grammatical structure of complex sentences.