Tell whether the following sets are closed under the operation: Numbers between 0 and 10, under taking their average Select an answer The positive real numbers under addition. Select an answer The set of 3x3 upper triangular matrices. Select an answer The positive real numbers under subtraction. Select an answer Vectors (x,y) in the plane with x>2, under matrix addition. Select an answer Prime numbers under addition Select an answer Polynomials with degree exactly 3, under addition Select an