concentrated nutrients and a steep salt gradient abundant shelter and concentrated nutrients a steep salt gradient and shallow water shallow water and abundant shelter
I gave my students this multiple choice question: 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?

Only one answer is correct among these 4 choices:
- concentrated nutrients and a steep salt gradient
- abundant shelter and concentrated nutrients
- a steep salt gradient and shallow water
- shallow water and abundant shelter

Could you tell me which one is correct?

abundant shelter and concentrated nutrients