What is the Hindley-Milner let-polymorphism and its restriction?

A Polymorphism allowing let-bindings to be polymorphic while lambdas are monomorphic B Only let-bindings (not lambda parameters) can be polymorphic in HM, preventing the system from requiring rank-2 types for lambda-bound variables C A restriction preventing polymorphism inside let bodies D A polymorphism rule for all binding constructs equally