Kara has 21 collectible buttons to sell. she sells the small buttons for $3 and the large buttons for $4, and earns a total of $68. if the small buttons are represented by x and the large buttons are represented by y, which of the following systems of equations can be used to calculate the number of large and small buttons sold?
a.) x + y = 21, 3x + 4y = 68
b.) x + y = 68, 3x + 4y = 21
c.) x + y = 21, 4x + 3y = 68
d.) x + y = 68, 4x + 3y = 21