Monosaccharides are polyhydric aldehydes and ketones which cannot be hydrolysed into simpler carbohydrates. The monosaccharides containing `-CHO` group are called aldoses while those containing `C=O` group are called ketoses. The aldehyde group is always present at `C_(1)` while keto group is generally present at `C_(2)`. All monosaccharides are oxidised by Tollen's reagent and Fehling solution and are called reducing sugars.
The monosaccharide molecules may be assigned D and L-configurations depending upon whether the configuration of the molecule is related to D- or L-glyceraldehyde. If the `-OH` group is attached to the carbon adjacent to the `-CH_(2)OH` group (last chiral carbon) is on the right hand side, it is assigned D-configuration. The molecule is assigned L-configuration if the -OH group attached to the carbon adjacent to the `-CH_(2)OH` group is on the left. The monosaccharides contain one or more chiral carbon atoms. Pentoses and hexoses have cyclic structures furanose (five membered) and pyranose (six membered). During cyclization, `C_(1)` in aldohexoses and `C_(2)` in fructose become chiral and the newly formed -OH group may be either on the left or on the right in Fischer projection formulae. These monosaccharides, therefore, exist in two stereoisomeric forms called `alpha`-anomer and `beta`-anomer while `C_(1) and C_(2)` are called glycosidic or anomeric carbon. The bonds joining glycosidic carbon are called glycosidic linkages. `D(+)` glucose exists in two stereoisomeric forms, `alpha`-D- glucose and `beta`-D-glucose. When either of these two forms of glucose i.e., `alpha-D-`glucose are dissolved in water and allowed to stand, these get slowly converted into other form and an equilibrium mixture of both is formed. This process is called mutarotation.
Which of the following pairs give positive Tollen's test?

Options

A. Glucose , sucrose
B. Glucose , fructose
C. Hexanal , acetophenone
D. Fructose, sucrose.

Answer

B