In solution, glucose exists as _________
(a) the open-chain form only
(b) the cyclic hemiacetal form only
(c) the cyclic acetal form only
(d) an equilibrium mixture of the open-chain form and cyclic hemiacetal forms
(a) the open-chain form only
(b) the cyclic hemiacetal form only
(c) the cyclic acetal form only
(d) an equilibrium mixture of the open-chain form and cyclic hemiacetal forms