Explanation:

- Liquidity trap: This occurs when interest rates are at their lowest, making monetary policy ineffective. People hold onto cash, rendering monetary policy tools like lowering interest rates ineffective. Demand for money becomes perfectly elastic.

- Cash crunch: A scenario where there is a short supply of money and credit, making it difficult for individuals and businesses to meet their financial needs.

- Infinite supply: Not a standard term related to bond markets or interest rates; suggests no limit in the availability of a resource which doesn’t directly apply here.

- Zero money velocity: This would imply money is not circulating in the economy, but it doesn’t specifically relate to the bond market or low interest rates.

Liquidity trap is the correct answer.