input: Write a python function to extract students with grade 'B' from student_dict.
  assert extract_students_with_grade_b({'john': {'grade': 'A', 'age': 15}, 'jane': {'grade': 'B', 'age': 12}, 'dave': {'grade': 'B', 'age': 10}})=={'jane': {'grade': 'B', 'age': 12}, 'dave': {'grade': 'B', 'age': 10}}
  assert extract_students_with_grade_b({'alice': {'grade': 'C', 'age': 14}, 'bob': {'grade': 'B', 'age': 13}})=={'bob': {'grade': 'B', 'age': 13}}
  assert extract_students_with_grade_b({'mike': {'grade': 'A', 'age': 17}, 'lucas': {'grade': 'A', 'age': 16}})=={}
output: def extract_students_with_grade_b(student_dict):
      return {name: info for name, info in student_dict.items() if info['grade'] == 'B'}