If L is a regular language over Σ = {a,b}, which one of the following languages is NOT regular?

Ten freiends planned to share equally the cost of buying a gift for their teacher. When two of them decided not to contribute, each of the other friends had to pay Rs 150 more. The cost of the gift was Rs. ___________.

A court is to a judge as _______ is to a teacher.

The search engine's business model _____ around the fulcrum of trust.