A student receives three unlabeled crystalline samples and records the following observations:
• Sample X scratches quartz and is an electrical insulator.
• Sample Y leaves a gray mark on paper and conducts electricity only along one crystal axis.
• Sample Z melts at 1135∘C and conducts electricity equally well in all directions.
Which identification of the samples is most reasonable?
a) Sample X is quartz, Sample Y is diamond, Sample Z is graphite.
b) Sample X is diamond, Sample Y is graphite, Sample Z is quartz.
c) Sample X is diamond, Sample Y is graphite, Sample Z is copper.
d) Sample X is diamond, Sample Y is copper, Sample Z is graphite.