Definition: Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

Positive Example 1 -
Input: fear of needles (medically known as belomophobia) is a term used to describe a large fear (phobia) of medical procedures involving needles.
Output: fear of needles

Positive Example 2 -
Input: the 2015 ping an chinese football association super league (chinese: 2015中国平安中国足球协会超级联赛) was the 12th chinese super league season and was won by guangzhou evergrande taobao.
Output: 2015 chinese super league

Negative Example 1 -
Input: roaring lion (march 11, 2015 – august 23, 2019) was an american-bred, british-trained thoroughbred racehorse.
Output: roaring racehorse

Negative Example 2 -
Input: the maillard reaction is a chemical reaction between an amino acid and a reducing sugar that gives browned food its distinctive flavor.
Output: chemical reaction

Now complete the following example -
Input: in physics, a gravitational field is a model used to explain the influence that a massive body has.
Output: 


Output: gravitational field