Astronauts<span> must </span>wear<span> spacesuits whenever they leave a spacecraft and are exposed to the environment of </span>space<span>. In </span>space, there is no air to breath and no air pressure.Space<span> is extremely cold and filled with dangerous radiation. Without protection, an</span>astronaut<span> would quickly die in </span>space<span>.</span>
Is there something else that goes along with the question?
Answer:
Hydrogen(H) and Heluim(He)
Explanation:
These are the only two valennce electrons and 1 energy levels.
Answer : The correct option is, (b) 22.5 M
Explanation : Given,
Mass of calcium nitrate tetrahydrate = 266 g
Molar mass of calcium nitrate tetrahydrate = 236.15 g/mole
Volume of solution = 
Molarity : It is defined as the moles of solute present in one liter of solution.
Formula used :

Now put all the given values in this formula, we get:

As calcium nitrate tetrahydrate dissociate to give 1 mole of calcium ion, 2 moles of nitrate ion and 4 moles of water.
The concentration of nitrate ion = 
Thus, the concentration of nitrate ion is, 22.5 M