In late childhood period children understand the conversion of which physical entity (ies)? [RTET-2011-I]

In late childhood period children understand the conversion of which

1.	In late childhood period children understand the conversion of which physical entity (ies)? [RTET-2011-I]
A.	Mass
B.	Mass and number
C.	Number
D.	Mass, number and area
Answer» E.