Classify the following sets as empty, singleton, finite or infinite sets:

(a) A = { }

(b) B = {50}

(c) C = {1, 4, 9, 16, 25}

(d) D = {x : x is a natural number greater than 20}

(e) E = {x : x is a negative whole number}

Find the ratio of 450 mL to 1.8 L in simplest form. Are the ratios 6 : 8 and 15 : 20 in proportion? Justify your answer.

Write any two binomials using the variables x and y. Also, identify their terms.