这题彻底晕菜,看manhattan的Ron的回复写着 my best response to this is "they make the rules, so just learn this as "incorrect" and save it in your memory for future reference."
不晓得可不可以这样理解咧 by 1997,children spent six hours a week doing household chores,here,“six hours” means an average figure during 1981 to 1997 but “in 1997”,that figure means a specific number in that year