Secondary explosives, like dynamite, require what to initiate detonation?

Secondary explosives, such as dynamite, are designed to be initiated by a shock wave or blast wave produced by a primary explosive. Primary explosives are very sensitive to heat, friction, and impact and can detonate with relatively minimal force. In contrast, secondary explosives require a stronger stimulus to initiate detonation, making them more stable and safer to handle under normal conditions.

When a primary explosive detonates, it creates a powerful shock wave that can then trigger the detonation of the secondary explosive. This method of initiation is crucial in controlled demolition and various military applications, ensuring that secondary explosives do not explode accidentally from minor impacts or heat sources.

The other choices do not apply to the initiation of secondary explosives. Electric current, direct exposure to flame, and manual ignition would not provide the necessary energy to detonate secondary explosives on their own. Thus, the correct answer highlights the reliance on a blast wave from another explosive to initiate detonation.