Which of the following is a subset of {b, c, d}?

{ }
{a}
{1 , 2 , 3}
{a, b, c}

What is the value of the expression below? (81^2)^1/8

Arley is building a model of a city map. in one part of the city, three roads form a right triangle, which harley draws as triangle abc, with the following measures: m∠b=90° and m∠a=30°. in his scale model, the hypotenuse of triangle abc, ac¯¯¯¯¯¯¯¯, has a length of 817−−√ cm. what is the value of a (the length of bc¯¯¯¯¯¯¯¯)?

If the scale factor between two circles is 2x/5y what is the ratio of their areas?