【单选题】

D　　Yellowstone Weather　　Yellowstone National Park is at high altitude(海拔). Most of the park is above 7,500 feet (2,275 meters).　　Yellowstone’s weather is unpredictable. In summer, it may be warm and sunny with temperatures in the high 70s.At night in any given month, the temperature may drop close to freezing. So it is best to come prepared for cold evenings and mornings, especially if you are camping or hiking. When you leave your campsite, please leave it prepared for possible thundershowers and winD、　　

A、sunny warm day may become fiercely stormy with wind, rain, sleet and sometimes snow. Without enough clothing, an easy hike or boat trip can turn into a battle for survival.　　Seasonal Weather InformationSpring　　Cold and snow continue into May, although temperatures gradually climB、Early in spring, daytime temperatures average in the 40s to 50s; by late May and June, they may reach the 60s and 70s. Nighttime lows fall below freezing.　　Summer　　Daytime temperatures are usually in the 70s, occasionally reaching the 80s in the lower elevations (高度). Nights are cool, temperatures may drop in the 40s and 30s-sometimes even the 20s. June can be cool and rainy; July andAugust tend to be somewhat drier, although afternoon thundershowers are common.Fall　　Weather can be pleasant, although temperatures average 10-20 degrees lower than summer readings. Nighttime lows can fall into the teens and lower. Snowstorms increase in frequency as the weeks go by or towards the end of the fall season.Winter　　Temperatures often stay near zero throughout the day, occasionally reaching high in the 20s. Suhzero nighttime lows are common.Annual snowfall averages nearly 150 inches in most of the park.At higher places, 200-400 inches of snow have been recordeD、 What does the underlined word "readings" refer to "_____"
A、literary works
B．books
C．announcements
D．figures