Which of the following is NOT representative of the mutual information?

Select one:

a$.$ I$($X; Y$)=$ H$($X$)-$ H$($X l Y$)$

b$.$ I$($X; Y$)=$ H$($X$)+$ H$($Y$)-$ H$($X$,$ Y$)$

c$.$ I$($X; Y$)=$ H$($Y$)-$ H$($X l Y$)$

d$.$ I$($X; Y$)=$ I$($Y; X$)$