A rock drawing in Spain from about 7000 B. C. shows a man who has climbed a hillside, found a crevice holding a hive, and is reaching in to grab the honey. Indeed, a lucky wanderer in just about any part of Europe, Africa, or Asia that wasn’t covered with ice could stumble on a hive and—at the risk of some stings—come away with a treat. (People in the Americas had no bees, so used syrups made from maple trees, agave cactus, or mashed fruits for their sweeteners.) Then someone figured out that you didn't have to be lucky. You could hollow out a log near bees, and they would make it their home. You could “keep” bees—you didn’t have to find them. –Sugar Changed the World,
Marc Aronson and Marina Budhos

Which excerpt from the passage provides evidence of a problem-and-solution structure?

“A rock drawing in Spain . . . shows a man . . . holding a hive.”
“A lucky wanderer . . . could stumble on a hive.”
“People in the Americas had no bees.”
“You could hollow out a log near bees, and they would make it their home.”