Which of the following is/are periodic?
(a) f(x)={1,0,if x is rationalif x is irrational
(b) f(x)={x−[x],21,2n≤x<2n+12n+1≤x<2n+2 , where [x] denotes the greatest integer function
(c) f(x)=(−1)[x] , where [x] denotes the greatest integer function
(d) f(x)=ax−[ax+a]+tan(2πx) , where [x] denotes the greatest integer function