Look it up why do you ask us look it up please look up
The correct answer is the cluster sample is obtained by selecting individuals who are easily contracted rather than who voluntarily respond because cluster sampling is a way of dividing a group then the individual responsible for picking from the selected population will select people from the cluster in which is easily contacted.
trees do not fix N2 from air they fix oxygen with the help of N2 fixing bacteria . so trees do not fix N2