Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions.
Sandburg received the Pulitzer Prize for his Collected Poems in

A．it helps strengthen his muscles.
B．it is good for a sound sleep.
C．it helps develop mental toughness.
D．it helps to lose weight.

A．Education in the U.S.
B．The school system in the U.S.
C．Summer holidays in the U.S.
D．Year-round schooling in the U.S.