It was suggested that Tom studied the document more thoroughly before attempting to pass72/80Mark the letter A, B, C or D on your answer sheet to indicate the underlined part that needs correction in each of the following questions. It was suggested that Tom studied the document more thoroughly before attempting to pass wasstudiedthoroughlyattempting toGiải thíchĐáp án là B. studied => had studied, vì QKHT + before + QKĐG/ V-ing