Which of the following expressions are polynomials in one variable and which are not?
State reasons for your answer.
\text { (i) } 4 x^{2}-3 x+7
\text { (ii) } \quad y^{2}+\sqrt{2}
\text { (iii) } \quad 3 \sqrt{t}+t \sqrt{2}
\text { (iv) } \quad y+\frac{2}{y}
\text { (v) } x^{10}+y^{3}+t^{50}