For the following choices name the sampling method chosen and explain if the chosen method is sound or flawed:

a. In a survey on COVID-19 vaccinations, the British Central Bureau of Statistics randomly selected and mailed 1700 teens (aged 12-17) about their vaccination status.

b. In another survey on COVID-19 vaccinations, the British Central Bureau of Statistics randomly selected 15 high schools in The UK and asked all of their students about their vaccination status.

c. In an online poll conducted by a British online newspaper, 4002 Internet users chose to respond, and 71% of them stated that they were fully vaccinated against COVID-19.