Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.

Let me give you an example: Passage: Martin Luther married Katharina von Bora, one of 12 nuns he had helped escape from the Nimbschen Cistercian convent in April 1523, when he arranged for them to be smuggled out in herring barrels. Suddenly, and while I was occupied with far different thoughts, he wrote to Wenceslaus Link, “the Lord has plunged me into marriage. At the time of their marriage, Katharina was 26 years old and Luther was 41 years old. Question: In a letter who did Luther credit for his union with Katharina?
The answer to this example can be: the Lord
Here is why: The paragraph clearly states that, Luther has credited the Lord for his union with Katharina, hence the Lord is correct answer.

OK. solve this:
Passage: The Doctor rarely travels alone and often brings one or more companions to share these adventures. His companions are usually humans, as he has found a fascination with planet Earth. He often finds events that pique his curiosity as he tries to prevent evil forces from harming innocent people or changing history, using only his ingenuity and minimal resources, such as his versatile sonic screwdriver. As a Time Lord, the Doctor has the ability to regenerate when his body is mortally damaged, taking on a new appearance and personality. The Doctor has gained numerous reoccurring enemies during his travels, including the Daleks, the Cybermen, and the Master, another renegade Time Lord. Question: How often does Doctor Who travel by himself?
Answer:
rarely