问题如下：
Which of various Greek measures can be added together across different currencies?
选项：
A. Delta
B. Gamma
C. Theta
D. None of the above
解释：
D is correct..
because Greeks for example do not imply the same level of risk across markets (e.g., delta in foreign exchange versus commodity markets).
老师你好，能解释下这题的意思吗？
