20. Generalizing exercise 8, show that the following sets in R2 have measure 0, which means that the set can be covered by a collection of balls with total area as small as we choose.

(a) The ‘‘integer lattice’’: fðn;mÞ j n;m A Zg

(b) 1 n ; 1 m

 

j n;m A Z; n;m00

 

(c) fðq; rÞ j q; r A Qg