Select the correct statement.
a) A single cell, Spirulina, can produce large quantities of food rich in protein, minerals, vitamins etc.
b) Body weight-wise, the micro-organism, Methylophilus methylotrophus, may be able to produce several times more proteins than the cows per day.
c) Common button mushrooms are a very rich source of vitamin C.
d) A rice variety has been developed which is very rich in calcium.