OPT:
A). earn more money by making advertisements in the carriage;
B). attract more tourists to come to Beijing to take the subway;
C). make the idea of "Green Travel" go deep into people's mind;
D). offer the passengers an additional reading platform;
Given those answer options, answer the question: Read the following article and answer the question. Article: BEIJING, (Xinhua) --China's capital city launched its first underground library, "M Subway Library" on Jan. 12,2015. The theme of its first activity is "Our Characters". Citizens riding the special train on subway Line 4 now can read e-books provided by the National Library by scanning the QR code in the carriage. "M Subway Library" is a public welfare program initiated by the Beijing MTR and the National Library to provide qualified book resources to the public through the platform of public transportation. The library will organize different themed activities regularly and recommend a dozen free books to the public each year in the long term. XINING, (Xinhua) --A bullet train made its maiden trip from Xining city to Lanzhou city in Northwest China on Dec. 26, 2014, marking the start of the full operations of the Lanxin high-speed railway. The Lanxin high-speed railway links Urumqi in the Xinjiang Uygur Autonomous Region with Xining, capital of Qinghai Province, and Lanzhou, capital of Gansu Province, slashing the total travel time from more than 23 hours to 11 hours and 50 minutes. The 1,776-kilometer high-speed rail line, with a designed speed of 250 kilometers per hour, is China's first high-speed railway built in a high-altitude region. NINGBO, (Xinhua) --Ningbo Subway Line 1 started running special trains with 3D paintings featuring the city's culture starting on Dec. 20, 2014.Through the paintings, the city in East China's Zhejiang province seeks to share its culture and attract more outside visitors. The paintings focus on four themes: rice cake, fishing festival, books and Buddhism. Question: The purpose of launching "M Subway Library" is to _ . Answer:
A:
D).