Put options with strikes of 70, 75, and 80 have option premiums of 4.00, 8.00, and 11.00, respectively. Given these information, in which way can you undertake an arbitrage?

I know the answer should be B, please show me how to get that so I can answer similar problems on my own.

(a) Buy a P(70), Short a P(75)

(b) Buy a P(70) and a P(80), Short two P(75)

(c) Short a P(75), Buy a P(80)

(d) It is impossible to make any arbitrage.