()是研究者通过感官或一定的仪器设备，有目的、有计划地观察儿童的心理和行为表现的方法，是早期儿童发展研究中常用的一种方法。

Time goes by so fast.  We must never miss the chance to show

love for our parents and make them _____ how much they mean to us.[ ]

A. to know

B. knowing

C. know