其它
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 $4b$ 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Equivalent Substitution->Algebraic Expressions
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. 