Assume that the universal set is r. consider the following sentence: .9t 2 r/ .t x d 20/ : (a) explain why this sentence is an open sentence and not a statement. (b) if 5 is substituted for x, is the resulting sentence a statement? if it is a statement, is the statement true or false? (c) if is substituted for x, is the resulting sentence a statement? if it is a statement, is the statement true or false? (d) if 0 is substituted for x, is the resulting sentence a statement? if it is a statement, is the statement true or false?