>>> from marks import best_students

>>> l = [['44444444D',8.3],['88888888H',10.0],['33333333C',10.0],['22222222B',4.8],['66666666F',5.3],['99999999J',10.0]]
>>> ld = best_students(l)
>>> ld
['33333333C', '88888888H', '99999999J']
>>> l = [['3A', 10]]
>>> ld = best_students(l)
>>> ld
['3A']
>>> l = [['3A', 5]]
>>> ld = best_students(l)
>>> ld
[]