Which of the following is a binomial?

1. a² - 6b² + 1000
2. -7x²y + xy² + 3x²y
3. ma²b - (m + n)a²b² + nab²
4. st² - st + 1

The sum of the polynomials bn - 8am - 8cp, 5cp - am + 2bn and 6am + 5bn - 3cp is equal to

1. -2am + 2bn - 4cp
2. -2am + 8bn - 2cp
3. -3am + 2bn - 4cp
4. -3am + 8bn - 6cp