Four species are listed below
i. HCO3⁻
ii. H₃O⁺
iii. HSO₄⁻
iv. HSO₃F
Which one of the following is the correct sequence of their acid strength?

•  iv < ii < iii < i

• ii < iii < i < iv

• i < iii < ii < iv

• iii < i < iv < ii