Rationale: An equation is a mathematical statement that asserts the equality of two expressions.
Option A, "x = 2," is an equation as it equates the variable x to the number 2.
Option C, "3x - 4 = 20y^2," is also an equation because it sets the expression on the left equal to the expression on the right.
Option D, "13y = y + 1," qualifies as an equation since it shows the equality between two expressions involving the variable y.
Option E, "x/5 = 9," is an equation as it equates the expression x/5 to the number 9.
Option B, "x² + xy - y," is not an equation; it lacks an equality sign, making it merely an expression.
