What does the modal in each express?

It might be a good idea to teach others about endangered species.

• Advice

• Obligation

• Prohibition

Visitors don't have to pay money to get in. This zoo is free.

• Not an obligation

• Not a prohibition

• Advice

In my country, hunters mustn't go near any endangered animals.

• Advice

• Prohibition

• Obligation

Humans have to protect animals in order for them to survive.

• Obligation

• Prohibition

• Advice

You ought to stop going to zoos if you don't agree with them.  

• Advice

• Obligation

• Prohibition