Nina is an artist who sells paintings online. She charges the same amount to ship each painting. When she sells 4 paintings, she charges a total of $9.96 for shipping. When she sells 8 paintings, she charges a total of $19.92 for shipping. How much more does Nina charge for shipping 20 paintings than for shipping 16 paintings?
a$2.49
b$9.96
c$19.92
d$29.96​