7. In the following expressions, - represents negation of a variable. For example, x stands for "NOT x"), + represents logical OR, and juxtaposition represents logical AND (e.g., (x+y)(y+z) represents (x OR y) AND (y OR z). Identify the expression that is satisfiable, from the list below. a) (y+z)(y+z)(y+z)(y+z) b) (y)(y+z)(y+z)(z+x) c) (y)(y+z)(z+x)(x+y) d) (y+z)(z+x)(y)(x+y)