A 30-item math test was graded using following procedure: a correct response was scored as +1, a blank response was scored 0, and incorrect response was scored -1. The maximum possible test score was 30; the lowest score possible was -30. The standard deviation of the test scores for the class was reported to be -2.13. Therefore
(a) Most students received positive scores
(b) A calculation error was made in determining the standard deviation
(c) Some students received negatives scores
(d) The class performed poorly on this test
(e) The test was too hard for this class