How many of the following statements are correct?
i) High concentration of $S{O}_2$ leads to stiffness of flower buds which eventually fall off from plants
ii) Higher concentration of $N{O}_2$  damage the leaves of plants and retard the rate of photosynthesis
iii) In pregnant women who have the habit of smoking the increased CO level in blood may induce premature birth, spontaneous abortions and deformed babies.
iv) The increased amount of $C{O}_2$  in the air is mainly responsible for global warming
v) Acid rain is harmful for agriculture, trees and plants as it dissolved and washes away nutrients needed for their growth
vi) Classical smog is also called as oxidizing smog and photo chemical smog is also called as reducing smog
vii) The $F^{-}$ ions make the enamel on teeth much harder by converting hydroxyl apatite into much harder flourapatite