In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.
One example: Article: There are a lot of public libraries in Britain. These libraries have more than one hundred and fifteen million books, not including those libraries of public schools. Public libraries not only lend books, music records but also help children, patients in hospitals and others to get special play readings, film shows and so on. The first public libraries were founded in about the year 1850. Today's public libraries usually have a reading-room, a lending room and a reference room. The greatest and most famous library in Britain is that of the British Library in London. It was founded in 1973 and has over 10, 000, 000 books. It has nearly every magazine, newspaper and book published in Britain and other parts across the world.
Answer: 10,000,000 or less.
Solution is here: How many books are there in the British Library?
Explanation: In the Article, 10,000,000 is mentioned for the number of books available in the British Library, so this is a good question.

Now, solve this: Article: On April 20,2013, a big and powerful earthquake attacked Ya'an, Sichuan Province. A great many buildings and houses _ a lot of people were killed and even more people lost their homes in the earthquake . Death toll from the 7.0-magnitude earthquake that hit Ya'an in southwestern China's Sichuan Province has risen to 188 (as of 18:00 pm, April 22, ), according to latest calculations by local Civil Affairs Bureau. At least 11,826 were reportedly injured in the quake while 25 are still missing, the bureau said. Experts are anticipating much less casualties in the quake than that from the 8.0 magnitude quake hit Wenchuan on May 12, 2008, also in Sichuan Province, which killed more than 80,000 and injured over 300,000. Energy released from Wenchuan earthquake would be 30 times of that from Ya'an quake, according to Zhang Xiaonan, an expert with China's Geophysical Union. Dai Junwu with Harbin-based seismological bureau expects the death toll at less than 1,000, and economic losses from the quake could reach up to 100 billion yuan. Police and expert teams have already rushed for rescue in the quick stricken areas, and the State Council has banned unauthorized rescue efforts and individual vehicles from going to the quake zone to avoid congestion . Rescue efforts were carried "timely and orderly", said Chinese Premier Li Keqiang who flew to the quicken-stricken area on Sunday, the second day of the quake. He was quoted as saying that "the current priority is to save lives".
Answer: April 20,2013
Solution:
When did Ya'an earthquake occur?