The boy often ________ a kite on Sundays in spring.
A:fly
B:flying
C:flies
D:flew
人体呼吸道表面的黏膜和纤毛都有阻挡病原体进入人体的作用.
冬天下雪后,室外的气温是﹣8℃,读做“零下8 ____”;在此温度下,堆的雪人会逐渐变小,这是 ____现象.