Atype of junction between cells that allows the passage of small molecules and ions from cell to cell is a

Gap junction

Explanation:

They are also called nexus or Macula communicans

These are specialized inter-cellular connections between a group of cells that directly connects the cytoplasm of two cells allowing passage of small molecules, ion and electric impulses.

They occur in almost all cells except sperm cell, erythrocytes and fully developed skeletal muscles. Gap junction are not found in simpler organisms such as sponges and slime molds.

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.