Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 44 to 50.

When trying to memorise new things, it’s easy to assume that the more work we put in, the better we will perform. But just try dimming the lights and enjoying 10-15 minutes of silence, and you may find that your memory of what you have learnt is far better. Although it’s already well known that we should sometimes rest while studying, new research suggests that we should aim for “minimal interference” during these breaks–avoiding any activity that could interrupt memory formation like checking your emails or surfing the Internet. The remarkable memory-boosting benefits of undisturbed rest were first documented in 1900 by the German psychologist Georg Elias Muller. In one experiment, Muller first asked their participants to learn a list of meaningless syllables. Following a short study period, half the group were immediately given a second list to learn while the rest were given a short break before continuing. In the end, the participants given the break remembered nearly 50 percent of the 2 lists, compared to an average of 28 percent for the other group. More recent research has also replicated Muller's finding in many different contexts. For example, a study found that these short periods of rest can also improve our spatial memory, for instance – helping people better recall the location of different landmarks. Crucially, this advantage lasts for a week after the original learning task, and seems to benefit participants of all ages. And in all cases, they simply let their minds wander in a quiet room without distraction when they rest. However, we should be careful as we daydream. This is because in another experiment, participants were asked to imagine a past or future event during their break, and this appeared to reduce their later recall of new material. So it may be safest to avoid any concerted mental effort as we rest. How this effect works is still not clearly understood, though some clues come from a growing understanding of memory formation. It is now well accepted that once memories are initially encoded, they pass through a period of consolidation that slowly transfers them into your long-term memory. This was once thought to happen primarily during sleep, with heightened communication between the hippocampus – where memories are first formed – and the cortex, starting a process that quickly builds and strengthens the neural connections needed for later recall. But it seems that comparable brain activity can occur during periods of wakeful rest as well. The next time you need to study a large amount of information for an exam, it's worth remembering that just like your smartphones, your brain also needs a regular periods of recharging to work properly. After all, even slight improvements in your memory could make the vital difference between a grade or two. (Adapted from bbc.com)

The word “optimal” in paragraph 4 is closest in meaning to _______ . A. efficient B. useful C. ideal D. valuable