Private inheritance VS composition : when to use which?

Private inheritance VS composition.

I'm having a little confusion when to use each. Since private inheritance seals, in a way, the chain on inheritance, given:

class A
{
private:
    int z;
protected:
    int y;
public:
    int x;
};

class B : private A
{
    /* B's data members and methods */
    /* B has access only to A's public and protected */
};

class C : public B
{
    /* can access no fields of B */
};

C won't be able to use any of B's fields. When would I use private inheritance, and when would I use composition?

thanks!