Which of the following is the multiplier (k) in an open economy (with no government sector)?

Which of the following is the multiplier (k) in an open economy (with no government sector)? O k = 1/(c + m) O k = 1/(1-c-m) O k = 1/[(1+c) - m O k = 1/(1 - m) O k = 1/[(1-c)+m]