Here's a problem to solve: Estuaries are called "the nurseries of the sea" because many species lay eggs and develop their young in estuaries. Which two features of estuaries support this role most?

Among the 4 following options, which is the correct answer?

- A: concentrated nutrients and a steep salt gradient
 
- B: abundant shelter and concentrated nutrients
 
- C: a steep salt gradient and shallow water
 
- D: shallow water and abundant shelter
 