Question 20: I am sure you all remember what a context-free grammar is. In case you have forgotten, a context-free grammar is a four-tuple (V, , R, S), where . V is a finite set whose elements are called vana . is a finite set whose elements are called terminals, . R is a finite set whose elements are called rules (or productions), and . S is an element of V and is called the start variable. I assume that the grammar G is in Chomsky Normal Form, which means that each rule in R has one of the following two forms: . A-+ BC for some A, B, C V. A afor some A E V and a E