Butter, lard and beef fat are all rich in unsaturated fats :)
If they both have Rr. The possibilities will be RR, Rr, Rr, rr. They have a 25% chance of inheriting the recessive trait instead of their dominant trait. That means that these traits are hidden and not expressed in the previous generation.
Answer:
Anything smaller or larger requires a higher water velocity to be eroded and entrained in the flow. The main reason for this is that small particles, and especially the tiny grains of clay, have a strong tendency to stick together, and so are difficult to erode from the stream bed.