根据以下内容，选择最恰当的答案。
The mystery(悬疑) stories of Sir Arthur Conan Doyle are famous all over the world.
The detective in his stories is called Sherlock Holmes. He solves mysteries in a most surprising way. Through it all, his assistant, Dr Watson, watches with admiration(钦佩). Watson then writes up the story of the mystery afterwards.
This is a useful story writing skill. Because Watson is not very clever, the story remains a mystery until the end. The readers can sometimes pick up some clues before he does! Then, to make Watson understand the mystery, Holmes has to explain it all, step by step. So we, the readers, get to see it step by step too. Watson is almost as good a character in the story as Sherlock Holmes!
The very first time they meet, Holmes greatly surprises Watson. When they are first introduced, Holmes says to Watson, "How do you do? I see you have been to Afghanistan.”
Watson is too amazed by this to ask Holmes how he knows this. Watson has, indeed, been to Afghanistan. Later, he asks Holmes about it. Holmes explains that there is nothing magical about what he does. He gets to know it all by very, very careful observation (观察).
Observation One
Watson was introduced to Holmes as a doctor. Yet Holmes thought that the way he stood and walked made him seem like a soldier. So, this meant he was probably an army doctor.
Observation Two
Watson's face was quite dark skinned. But his wrists(手腕) were pale. So his dark face was probably sunburnt. This meant that he had been to a hot, sunny country. As he was a working doctor, this was not a holiday. He must have gone with the army.
Observation Three
At the time, the British army had soldiers at bases in many parts of the world. So there were still a lot of places to choose from. However, Holmes saw that Watson looked very tired and he was always holding his left arm, as if it was painful. So, he had probably been with the army to a place where they were fighting. There was only one place where the British army was fighting at that time: Afghanistan.
