A regular die has six sides with numbers 1 to 6 marked on its sides. If a very large number of throws show the following frequencies of occurrence: 1 $\rightarrow$ 0.167; 2$\rightarrow$0.167; 3$\rightarrow$0.152; 4$\rightarrow$0.166; 5$\rightarrow$0.168; 6$\rightarrow$0.180. We call this die
(a) irregular (b) biased
(c) Gaussian (d) insufficient