What is the most difficult train to eatch?

what's the most difficult train to catch? what is the most difficult key to turn? What is the most difficult product sell to chinese What is the most difficult key to turn?Answer：donkey （驴、傻瓜） What is the most difficult part of riding the bike? what's the most diffcult train to catch? what is the most difficult part of studying science What do you think is the most difficult s____? what's the best most difficult to learn What kind of the passengers are the most difficult to serve 选择哪一个This is the most difficult That problem is the most difficult ---- all