b) . Both \( A \) and \( R \) true but \( R \) is not the correct explanation of \( A \)
c) \( A \) is true but \( R \) is false
d) \( A \) is false but \( R \) is true
17. A - Pure substance in which molecules are made-up of only one kind of atoms are known as elements.
\( R \)-hydrogen, oxygen and nitrogen are elements.
18. A - the outer membrane of mitochondria is folded into cristae. \( R \)-cristaeifreases the surface area.
19. A-All plant tissues divide throughout their life.
\( R \) - Meristematic tissues are localised in certain regions.
20. A-The velocity of a body is a vector quantity.
\( R \) - a vector quantity has only magnitude.
