Of the following choices, what would the BEST interval size be if you want to use 10 intervals?

a. 10

b. 25

c. 20

d. 5

e. 30

now here is the number sequence...

31 46 78 219 45 147 33 45 112 108 194 125 68 99 134 201 78 115 101 176

I think the answer is 20. My question is am I correct or am I going about it wrong