Please Explain

-A Worker's marginal rate of substitution of bananas for Carrots ,(MRS B,C)is 2, if this consumer gives up one banana, how many additional carrots will keep his utility constant? -Lets say corn syrup and sugar are perfect substitutes, In this regard is this hypothetical scenario true or false? The MRS or marginal rate of substitution on the indifference curve showing what this consumer prefers is diminishing. In case it is false please explain the justification -In what way is the market demand curve for a good (X) Calculated? to make it simpleJ pretend there are 5 consumers in a fictitious economy