When using timechart, how many fields can be listed after a by clause?

A、0,becausetimechartdoesn'tsupportusingabyclause.
B、1,because_timeisalreadyimpliedasthex-axis.
C、2,becauseonefieldwouldrepresentthex-axisandtheotherwouldrepresentthey-axis.
D、Thereisnolimitspecifictotimechart.