2. In Japanese, I like that you are "Hao きだよ" and romaji is "su ki da yo", taking su ki's sk, so many girls take sk as the meaning of liking you.
"sk" is also the pinyin abbreviation of "Happy Birthday", which means "SK" stands for Happy Birthday.
4. "sk" also means thanks, because the pronunciation of thanks is similar to sk, so sk also expresses thanks.