Even though the options were not stated in the question, it is clear that the prediction required must reflect the stated fact that the sublimation temperature of CO2 is -78.5°C. This is the fact that was observed in the cause of the experiment. Hence any of the options that reflects this statement must be the correct option among the list of options available in the question.
A spontaneous chemical process is one in which product formation is favoured under the specified conditions. A spontaneous process yields the product without an external input of energy.
A spontaneous reaction will always occur when ∆H is negative and ∆S is positive. These are the signs of enthalpy and and entropy change that gives rise to a spontaneous chemical process at all temperatures. Recall that a spontaneous process is one in which ∆G<0.
To answer the question that is: "which factors affect a river's load", we have to understand that all the things mentioned are important. All these options (river's slope, streambed shape and volume of flow) affect the amount of energy that the river has to spend and the way the river spend that energy, so, it is right to mark the alternative <span>d) all of the above.</span>