Look at the code below. You have four classes $($O$,$ X$,$ T$,$ and M$)$ and a variable of each of these.

O o;

X x;

T t;

M m;

The following assignments are all legal $($assume that they all compile$)$

m $=$ t;

m $=$ x;

o $=$ t;

The following assignments are all illegal $($they cause compiler errors$)$

o $=$ m;

o $=$ x;

x $=$ o;