What Is the Basic Process in Probability Proportionate to Size

What is the basic process in probability proportionate to size sampling?

A) give clusters all the same chance of being picked but pick a different number of elements per cluster
B) give bigger clusters a greater chance of being picked but then take the same number of elements per cluster
C) give bigger clusters a greater chance of being picked and then take proportionately fewer elements in the bigger clusters
D) give all clusters an equal chance of being picked and then take the same number of elements per cluster