Think about how the oyster mats might be harmful instead of helpful? oyster reef articles

Which property is used to identify the origin of an igneous rock? color density hardness crystal size

Which type of organism is missing from the food web seen in the illustration? primary consumers decomposers primary producers secondary consumers

Ialready know the answers to these questions and they are on the attachment. but i just need to know why that answer is the correct one. plz asap! this is for a report that i need to email by 10!