Which microorganism produces 200 g of protein in a single day?

Consider the following four statements $(A-D)$ and select the option which includes all the correct ones only.
$(A)$ Single cell $Spirulina$ can produce large quantities of food rich in protein,minerals,vitamins,etc.
$(B)$ Body weight-wise,the microorganism $Methylophilus$ $methylotrophus$ 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.

Why are proteins synthesised from Spirulina called single cell proteins?

The alga rich in protein is

$A$ person who is allergic to pulses was advised to take a capsule of $Spirulina$ daily. Give the reasons for the advice.

Which bacterium has the capacity to produce $25$ tonnes of protein due to its high rate of biomass production and growth?