QUESTION 3 Fill in the blank. A Shift-Reduce parser is essentially a state machine + O a. a set O b. a queue O c. a Recursive-Descent engine O d. a key-value store O e. a circular buffer O f. a stack QUESTION 4 Briefly, what does the SHIFT operation in a Shift-Reduce parser do? O a. Divide the top stack item by a power of 2. O b. Multiply the top stack item by a power of 2. O c. Run a production in reverse, popping its right-hand side and pushing its left-hand side. O d. Clear the stack. O e. Terminate parsing, moving the AST into the output register. Of. Push a symbol and advance to the next symbol