47. According to the passage, why is the study significant?
(A)The study was first to examine the effect of various amount of caffeine on nightly sleep.
(B)The study was first to include participants’ written sleep diaries to keep their sleep records.
(C)The study investigated the effect of caffeine on nightly sleep in specific times before bedtime.
(D)The study recruited volunteer males and females to participate in the experiment on caffeine consumption.