Observational assessment techniques have become much more sophisticated, reducing the limitations just presented. Film analysis of the infant's responses, heart and respiration rate monitors, and nonnutritive sucking devices are used as effective tools in understanding infant perception. ■Film analysis permits researchers to carefully study the infant's responses over and over and in slow motion. ■Precise measurements can be made of the length and frequency of the infant's attention between two stimuli. ■Heart and respiration monitors provide the investigator with the number of heartbeats or breaths taken when a new stimulus is presented. ■Numerical increases are used as quantifiable indicators of heightened interest in the new stimulus. Increases in nonnutritive sucking were first used as an assessment measure by researchers in 1969. They devised an apparatus that connected a baby's pacifier to a counting device. As stimuli were presented, changes in the infant's sucking behavior were recorded. Increases in the number of sucks were used as an indicator of the infant's attention to or preference for a given visual display.
Look at the four squares [■] that indicate where the following sentence could be added to the passage.
The repetition allows researchers to observe the infant's behavior until they reach agreement about the presence and the degree of the infant's response.
从这道题当中你可以看到指代词the的使用，也就是说它所指代的repetition应该前面提到过。Repetition这个词在中文里面是重复和反复的意思，所以你就看每个选项前面，那里提到了反复有关系的地方。结果你发现在B选项前面就有over and over，这个就是反复的意思。所以我们选B。