Consider the following statements. $S_1:$ The sequence of procedure calls corresponds to a preorder traversal of the activation tree. $S_2:$ The sequence of procedure returns corresponds to a postorder traversal of the activation tree. Which one of the following options is correct?

In compiler runtime environments:

• An activation tree represents the activations of procedures during execution, where each node represents an activation and child nodes represent procedures called by that activation.

• When a procedure is called, execution enters its activation before its sub-procedures are called. Therefore, the sequence of procedure calls corresponds to visiting the root before its children, which is a preorder traversal of the activation tree. Thus, statement $S_1$ is true.

• When a procedure finishes, all activations of the procedures it called must have already completed and returned. Therefore, a procedure returns only after all its children have completed, which corresponds to visiting all children before visiting the parent node, i.e., a postorder traversal of the activation tree. Thus, statement $S_2$ is true.

Hence, both $S_1$ and $S_2$ are true, which corresponds to option (C).