<u>The correct answer is </u><u>C</u>. Viajar en avión es más caro que viajar en tren. That means that traveling by plane is more expensive than traveling by train. The expression "más caro" means "more expensive". This is true: a plane ticket is much more expensive than a train one. The expression "más económico" means "cheaper", which is not true about planes compared to trains. And the expression "menos rápido" means "slower", which is also false since plane traveling is the faster of all.
