Read this excerpt from "The Unseen Values."

There are always those who will say it cannot be done. Even in this era of sophisticated flight there have been those who said the sound barrier would never be broken. It was. Others said later that space vehicles would never get through the heat barrier. They have. Now, some say men will never overcome the radiation barrier in space. But we can be sure they will.
Which of the following themes could be inferred from the passage above?

embracing freedom
making sacrifices
showing compassion
facing challenges