PLEASE HELP ASAP What term describes physical weathering due to collisions between sand and sediment carried in moving water and the rock surface over which the water flows?

A) Dissolving
B) Abrasion
C) Wedging
D) Erosion

I think it is either B or D, does anyone know for sure what the answer is?

The serengeti plains are part of the african savanna ecosystem and are home to a variety of different species of plants and animals. the serengeti plains experience a seven-month period of seasonal drought each year, during which the ecosystem receives only four inches of rain and the availability of some resources becomes very scarce. which type of limiting factors does the seasonal drought in the serengeti plains affect?