There are $50$ even number cards and $50$ odd number cards here. If she chooses odd+odd or even+even, she will get an even sum. But if she chooses odd+even or even+odd, she will get an odd sum. So each of the two probabilities is equal. 
Summer has $100$ number cards each with a different number from $0$ to $99$. What is the probability that when she chooses two cards randomly, the sum of those two is an even number? 

a. $\frac12$ 
b. $\frac13$ 
c. $\frac14$ 
d. $\frac16$ 