persistent

definition: 
Continuing firmly or obstinately in a course of action in spite of difficulty or opposition:
sentence: 
The attorney's persistent questioning weakened the witness.
difficulty: 
very easy
toefl frequency: 
3
toefl classification: 
adverb: 
adjective: 
adverb sentence: 
adjective sentence: 
verb: 
verb sentence: 
noun: 
persistence
noun sentence: 
His persistence earned him a spot on the team.