Do blood cells have a rigid of cells walls

All of the following are part of the biosphere except

Sally is making a model of a magnesium atom with an atomic mass number of 24 for her chemistry class. she has foam balls for the protons, neutrons, and electrons. she has added 6 neutrons to her model so far. how many more neutrons does she need to add to complete her neutral atom of magnesium?

How many grams of sodium chloride are in 250ml of a 2.5m naci solution