attempts, attempt, attempting, in, on, at All their ______ to re

attempts, attempt, attempting, in, on, at All their ______ to rescue the child from the burning building were_____ vain.
【正确答案】：attempts，in
【题目解析】：attempt可作不可数名词和可数名词，根据谓语were可判断为复数主语表示尝试，in vain是固定词组表示“徒劳地，无用”。