What recommendation specifies the point at which a nutrient is likely to become toxic?

The answer is UL, the tolerable upper intake level or as named as UL is the maximum typical daily consumption levels at which no risk of opposing health effects is predictable for most of the individuals in a detailed group based on stage of life. The risk of opposing reactions upsurges as intake surpasses the UL. 

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.