A woman makes small cakes from home. In one hour, she can produce 30 cakes, and she works for four hours each day. The ingredients cost a total of $\$50$, and each cake is sold for $\$2$. What is the opportunity cost if she chooses to take a working day as a holiday?
- A$\$50$
- B$\$240$
- C30 cakes
- D120 cakes