Which of the following statements are true? (you can select multiple)多项选择题

a. Aquifers are always confined by aquitards above and below.

b. Gravels and sands are typical aquifer materials.

c. Aquifers have high porosity and permeability, allowing water to flow easily.

d. Clay and mudrocks act as aquitards because they impede water flow.

e. Aquitards are impermeable and cannot store any water.