Which of the Following Statements Is True? Consider Options Written

Which of the following statements is true? Consider options written on a non-dividend-paying stock.

A) All European and American options have negative theta.
B) European options and American calls have negative theta,but American puts may have positive theta.
C) European options have negative theta,but all American options may have positive or negative theta.
D) Theta may be positive or negative for both American and European options.