How many double bond equivalents (degree of unsaturation) does a compound with the molecular formula $C_6H_{12}O_6$ possess?

Fructose is an example of :-

Which of the following statements is correct?

The number of $-OH$ groups in open chain and ring structures of $D$-glucose are respectively:

Which of the following pairs give a positive Tollen's test?

Match the structures in Row $I$ with their names in Row $II$.
Row $I$:
$(P)$ $\alpha-D-(+)-$Glucopyranose
$(Q)$ $\beta-D-(+)-$Glucopyranose
$(R)$ $\alpha-D-(-)-$Fructofuranose
$(S)$ $\beta-D-(-)-$Fructofuranose
Row $II$:
$(i)$ $\alpha-D-(-)-$Fructofuranose
$(ii)$ $\beta-D-(-)-$Fructofuranose
$(iii)$ $\alpha-D-(+)-$Glucopyranose
$(iv)$ $\beta-D-(+)-$Glucopyranose
Correct match is: