Detailed Instructions: 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.
Problem: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?
Solution:
rarely