Which of the following statements about amino acids are not true?

(a) They are low melting crystalline solids.

(b) Most naturally occurring amino acid have L-configuration.

(c) Isoelectric point of neutral amino acids lie at $\mathrm{pH}$ between $5.5$ to $6.3$.

(d) Glycine is the only naturally occurring $\mathrm{\alpha }$-amino acid which is optically active.