The following are the numbers of words per minute that a secretary typed on several occasions on four different typewriters

A: 71 75 69 77 61 72 71 78

B: 68 71 74 66 69 67 70 62

C: 75 70 81 73 78 72

D: 62 59 71 68 63 65 72 60 64

Assuming that the variance of the number of words of the four secretaries are equal, test at the 5% level of significance whether the differences among the four means can be attributed to chance.