True or False:
For given f(x,y) and given D subset of |R^2, f has a global minimum value on D.
A. f(x,y) = x^4 + y^6           D = |R^2
B. f(x,y) = x^2 + y^3           D = |R^2
C. f(x,y) = x^3 + y^3           D = {(x,y): x^2+y^2 <= 1}
D. f(x,y) = 1/(25 + x^2 + y^2)  D = |R^2

Answers:
T,F,T,F