Which of the following matrix calculations are well defined (the sizes match)? If the computation makes sense, perform it. 
( (a) ) ( AA ) 
( (b) ) ( BB ) 
( (c) ) ( CC ) 
( (d) ) ( DD ) 
a) ( (b) ) ( BB ) 
b) ( (a) ) ( AA ) 
c) ( (c) ) ( CC ) 
d) ( (d) ) ( DD )