Ten tickets numbered $1, 2, 3, ..., 10.$ Six tickets are selected at random one at a time with replacement. The probability of the largest number appearing on the selected ticket being 7 is
a. $(7^6 +1)/10^6$

b. $(7^6  6^6)/10^6$

c. $(7^6+6^6)/10^6$

d. $6^6/10^6$
