Which pronoun has an unclear antecedent in the following sentences?

The sixth-grade students liked volunteering their time to read to the elderly people in the hospital. It made them happy.
Their
It
Them
No unclear antecedents

Read the passage. the dust bowl during the 1930s, extreme drought and high winds made life incredibly difficult in the great plains region of the united states. in many areas, rainfall was forty percent less than normal. the soil became extremely dry, and when strong winds blew, they created tremendous dust storms. these so-called “black blizzards” were so thick that they made daylight seem like dusk. during this terrible period, the great plains region of the united states was named the “dust bowl.” over the next few years, the drought continued, and the winds created more dust storms. in some places, the drought and high winds lasted for eight years. many people wondered if the black blizzards would ever end. which statement about this paragraph’s main idea is true? this paragraph’s main idea is stated in the topic sentence. there are not enough details to support the main idea. this paragraph’s main idea is implied. this paragraph does not express a main idea.