Hi Junnan. In this case "still" would not be appropriate for the context. "Still" can be used in many ways, such as, "He still likes listening to heavy metal after all these years," or "We still have some birthday cake left if you want some." But in the above sentence, "still" is the first word. Whenever "still" is the first word in a sentence, it almost always means in contrast to what was said before; it functions in these cases like "but" or "however." Here's an example: "I know it's a little bit cold outside. Still, I'd like to go to the beach anyway." The transition that occurs from the first sentence to the second is one of contrast or opposition to what was said before. There is no such contrast between the two sentences you're asking about, so "so" is the only one that works.