Mollusks with two shells held together by muscles are called a. bivalves b. cephalopods c. gastropods d. cnidirapods

Explanation/

A valve is a part of a shell (a molluscs shell) that can articulate (move). Bivalves are are molluscs that have 2 of these articulated shells, called valves, are named bivalves:

Bi - 2

valves - articulated parts of a shell

Example:

Clams and mussels are both bivalves.

→ Hopefully the image below helps you get a deeper understanding of what bivalves are and look like. Image source: Pinterest.

Hope it helped,

BioTeacher101

b tumor cells have a cell wall instead of a membrane

explanation:

tumor cells different from normal cells because tumor cells have a cell wall instead of a membrane.

the earth is thought to have been formed about 4.6 billion years ago by collisions from a cloud of hydrogen, helium, rock, ice and other materials. these were pulled together by gravity to form objects in and around earth like- asteroids and small early planets called planetesimals.

so, the best possible answer to this question will be option d- to harden materials to form different parts of earth’s layers