Standard Form of Algebraic Expressions

Q. Which of the following is/are in standard form of an algebraic expression?

1. $15x^2+x^3+9x$
2. $9x^2+6x+15$
3. $7x^3+3x^{2}y+6xy+9x$
4. $6x^3+x^4+5x^2$

Q. Identify the expression that are in standard form?

1. $9x^3+6x^2+17xy+5$
2. $6x^4+2x^2+3xy+5x+6x^3$
3. $2+9x-x^2$
4. $9x^3+45x+3$

Q. Which of the following is the default operation in algebra, if no operation is specified between two variables?

1. Addition
2. Subtraction
3. Multiplication
4. Division