Please answer the following question: Read the following article and select the best answer. Article: San Francisco has many museums. You can visit one and learn something new. We have art, science, history and other special museums. Some museums are open every day. Some are only open on certain days. Many museums are expensive, and others are free. But some of the expensive ones have free days . Many museums have websites. The websites usually have information about the things you can see in the museums. On a museum's website you can get information about ticket prices and free days. You can also know where the museum is and when it is open. Sometimes the information is on the first page, and it's very easy for you to find. But sometimes you have to click on the words like Visit Information, or About to find the information. That will be hard work for you. Question: Some museums in San Francisco are open every day, but some are only open _ . - on weekdays - on weekends - on certain days - on holidays
Answer:
on certain days