Which of the following is a characteristic of a simple machine?

A simple machine is designed to make work easier by allowing us to apply a smaller amount of force over a longer distance or to change the direction of a force. The characteristic of being able to change the direction of a force is fundamental to devices such as pulleys or levers. For instance, a pulley allows you to pull down on a rope to lift an object upward, effectively altering the force's direction.

This ability to change the direction of force is crucial in many applications, making tasks less labor-intensive. It enables the use of gravitational energy or manual input in more efficient ways, thanks to its mechanical advantage.

The other characteristics mentioned in the question do not align with the primary features of simple machines. For example, simple machines can often operate under various conditions, not solely under gravity, and while they do require energy to function, this is not a defining characteristic unique to simple machines, as all machines require energy. Additionally, simple machines can frequently be used in conjunction with one another to form complex machines, enhancing their effectiveness and versatility, thereby contradicting the idea that they cannot be combined.