The product of 13 integer numbers is positive. From this intormation we can deduce that it is necessarily true that: a)If (a) all factors are positive. b) one factor is positive and 12 factors are negative. (с) 7 factors are positive and 6 factors are negative. (d) there is an odd number of positive factors (in the product), (е) at least two factors are negative. Which answer is correct and why ?