Which of the following is not a complex number: sum of complex numbers, difference of complex numbers, product of complex numbers

let z and y are two complect numbers such
that:

z= a + bi

y= c+
di

Now let us verify:

sum of
compelx numbers:

(a+ bi) + (c+di) = (a+c) +
(b+d)i

Then , the sum is a complex
number.

Now the
difference:

(a+ bi) - (c+di) = (a-c) +
(b-d)i

The difference is a complex
number.

Now the
product:

(a+bi)*(c+di) = (ac + bci + adi +
bi*di)

                     = ac + (bc+ad)i -
bd

                     = (ac-bd) +
(bc+ad)i

The product is a complex
number