Here's why:
* Two independent clauses: "Deep in thought" is a complete thought, and "Betty studied for her English exam" is also a complete thought.
* No conjunction or punctuation: There's no word like "and", "but", "or", "so", etc. to connect the two clauses, and there's no semicolon or period to separate them.
To fix it, you could:
* Add a conjunction: "Deep in thought, and Betty studied for her English exam."
* Add a semicolon: "Deep in thought; Betty studied for her English exam."
* Make it two separate sentences: "Deep in thought. Betty studied for her English exam."