Explanation:

In a zero volatility environment with an upward sloping yield curve:

• Upward sloping yield curve means longer-term rates are higher than shorter-term rates
• Zero volatility means the yield curve remains unchanged over time
• Bond valued near par suggests the coupon rate is close to current market rates

As time passes and the bond approaches maturity:

• The bond's remaining life shortens
• The relevant comparison rate moves down the yield curve to lower rates
• The bond becomes less likely to be called because the issuer would need to refinance at higher rates (due to the upward sloping curve)

Therefore, the call risk decreases over time as the bond approaches maturity.