Which of these machines are levers of second order?
Crowbar, tongs, wheel barrow, nutcrackers, lever balance

Explanation

To determine which of the listed machines are levers of the second order, we need to understand the definition of a second-order lever. In a second-order lever, the load is positioned between the effort (force applied) and the fulcrum (the pivot point).

### Step-by-Step Solution:

1. **Identify the Definition of a Second-Order Lever**:
- A second-order lever has the load located between the effort and the fulcrum.

2. **Analyze Each Option**:
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