exercise: bias differencediscrete time model vs. continuous time model

[fullofquestions]

When pricing an option with a discrete time model using the same volatility
assumption, as compared to a continuous time model, the value will tend to go:
a. Up
b. Down (ANS)
c. Up or down, according to the in-the-moneyness of the option
d. Up or down, no rules

Can anyone point me to a resource or example? I don't see why a discrete model would bias downward compared to a continuous model...