What challenges are currently facing the country?

Which sentence uses parallel structure? a. spring is a time for growth, renewal, and to rain. b. science provides us with some answers, some questions, and figuring out how our world works. c. as long as you use your best judgement, learn from your mistakes, and taking advice, you’ll grow as a person. d. once things are done, there is no need to dwell them, to worry about them, or to decide who to blame.