Like terms are terms that have the same variables. For each variable, the number of times it is multiplied by itself is also the same. 
How many of the following pairs of terms are like terms?  ($1$) $2$ and $6$;~($2$) $-2a$ and $-2c$;~($3$) $78x$ and $-200x$;~($4$) $8$ and $y$;~($5$) $4ab$ and $4b$ 
Options:
(1) $$1$$ 
(2) $$2$$ 
(3) $$3$$ 
(4) $$4$$ 