A. Two decades. B. Three decades. C. Four decades.

阅读理解（共两节，满分40分）第一节（共15小题：每小题2分，满分30分）

Snorkelling in Silfra

　　The clear water at this UNESCO world heritage(遗产) site in Thingvellir National Park will leave you speechless. It is one of the highest rated dive sites in the world. During this journey, you will snorkel within a crack between the Eurasian and American continents. You'll experience truly amazing views of the colorful underwater landscape, marine life(海洋生物), and geological formations. Silfra's water is fresh and drinkable at around 2°C -- 4°C all year round. Visibility can reach more than 100 meters, and swimming often feels more like flying.

　　This trip is for anyone who knows how to swim and is up for a unique adventure. We offer transportation with a pick-up in Reykjavik, and provide you with a dry suit. The total length of the tour is approximately five hours, and includes in-water snorkelling lasting around 30 minutes. We'll guide you through the main Silfra Big Crack, Silfra Hall, Silfra Cathedral and Silfra Lagoon.

　　Included:

　　★ All necessary snorkeling equipment;

　　★ Hot drinks and food;

　　★ Silfra entrance fee of 1,000 ISK.

　　Availability: All year at 09:00 and 15:00. Pick-up in one hour before.

　　Requirements:

　　Participants need to complete the Silfra medical statement before the tour.

　　Divers must:

　　★ Be at least 18 years old (young people under 18 years old need a signature from their legal guardian), and the maximum age is 65 years old;

　　★ Be able to communicate in English.

　　★ Be physically fit;

　　★ Not be pregnant;

★ Not be under the influence of alcohol or drugs;