After drying of a fruit juice, a number of factors need to be considered to ensure that the powdered version of the product remains microbiologically, physically, and chemically stable during storage. Select all of the appropriate statements below $($there may be more than one$).$

a$.$The water activity should be below a critical value to inhibit microbial growth

b$.$The glass transition temperature should be below a critical value to control physical state of the powder, thereby affecting the tendency for the powder to clump or flow smoothly, as well as the stability and evaporation of flavors

c$.$The moisture content should be above a critical level to ensure the powder is microbially stable

d$.$It may be necessary to add a "bulking agent" to the powder to replace the mass normally occupied by sugars