134. Read the following passage, then answer the question below.

Anyone who has tried to learn a second language in adulthood knows how difficult it is to lose a foreign accent. Such experiences inspired the critical period hypothesis, proposed in the 1960s, which holds that the brain is especially receptive to language during early childhood and gradually loses that flexibility around puberty. Support for the idea comes from tragic cases of children raised without human contact who, when found years later, never fully mastered grammar even after intensive instruction.

Yet the evidence is more complicated than the hypothesis suggests. A large study of nearly two thousand immigrants to the United States found that learners who arrived before age ten reached near-native grammar, as predicted, but the decline in ability after that age was gradual rather than abrupt. There was no sharp end point at puberty, only a slow decrease that continued into the late teens and beyond. The pattern implies that the notion of a single 'window' that slams shut may be misleading.

Researchers have proposed several alternative explanations. One is that older learners use different mental resources: whereas young children absorb language largely unconsciously, adolescents and adults rely on explicit rules and memory, which are slower and less reliable for sounds. Another is that the amount and quality of exposure matter at least as much as age. Children are usually immersed in the language all day, in schools and playgrounds, while adults may use it for only a few hours at work. When adult learners receive comparable immersion, some achieve proficiency that is difficult to tell from that of native speakers, though they remain a minority.

The debate has practical consequences. If age sets a hard limit, then starting language classes in early childhood is essential; if exposure is the main factor, then schools might better spend resources on increasing the hours of use at any age.

The word 'which' in the third paragraph (explicit rules and memory, which are slower) refers to