C. Planets can move at a varying speed due to forces exerted in space.
The molality is 0.54 M when 1.34 moles of NaCl is present in 2.47 kg of solvent.
<u>Explanation:</u>
Molality is the measure of how much of amount of solute is dissolved in the solvent. So it is calculated as the ratio of moles of solute to the grams of solvent.

As in this case, the solute is NaCl and solvent is unknown. So the moles of solute is given as 1.34 moles and the mass of solvent is given as 2.47 kg.
Hence, 
Thus, the molality is 0.54 M when 1.34 moles of NaCl is present in 2.47 kg of solvent.
A set that contain no element is called the EMPTY OR NULL SET and it is represented by { }. A set is defined as a well defined collection of objects. Elements of a set are those objects that belong to a set. So, an empty set do not have any object inside of it.