What time of a day can you think most quickly? Are you a morning person? Or does it take you a few hours to get your brain (大脑) going? A recent research suggests (表明) that we should pay attention to our body clock. It can tell us at what time of a day we can work best.

Many people work best later in the morning. This is because the body’s temperature takes time to rise through the day. But everyone’s body clock is different and some are special. Morning people will get up early and work better early in the day. Evening people will work later but work better towards the end of the day.

Research shows that we’re better at some activities at certain times of the day. Physical performance (体能) is at its best between three o’clock and six o’clock in the afternoon, so it’s better to exercise later in the day. Between noon and 4:00 pm, people begin to pay less attention. This is because we think less quickly after a big meal. Research also shows we become sleepy around 2:00 pm. This is why people in Spain take a short sleep in the early afternoon.

Finally, it is best to eat something during our activities. This can help our body to burn calories (卡路里) better and stops our blood sugar levels (血糖) from getting too high.

Historians study the past. They care about past events and people who lived in the past. But historians do not just learn names, dates and places. Instead they try to explain past events. They want to know why an event happened. They want to know why a person acted in a certain way. And they want to know how one event caused another to happen.

To learn about the past better, historians must study many sources. First, they use primary sources that were recorded at the same time as an event happened. They could be journals. They could be books. They could be newspaper articles or photographs. In modern times, they could even be video recordings. Historians use primary sources to get the opinions of eyewitnesses (目击者) to important events. They also use secondary sources. These are works written by people who did not see an event themselves. Good historians use both primary and secondary sources in their work.

There are many kinds of historians. Some historians like political history. Others study military history. Some are centered on economics. And others prefer social or cultural history. All of them are important. And all of them help us understand the past better.

A．

B．

C．

D．