Which type of explosives are generally characterized as being harder to initiate?

The correct answer is secondary explosives. Secondary explosives are designed to be more stable and less sensitive to initiation than primary explosives. This means that they require a stronger initiating device, such as a detonator or a primary explosive, to cause detonation. Their stability and insensitivity make them safer to handle and transport, but also indicate that they are generally harder to initiate.

In contrast, primary explosives are sensitive and can be easily triggered by heat, shock, or friction. Blasting agents are a type of secondary explosive that requires a significant initiation energy and cannot detonate under normal conditions. Low explosives, while not as sensitive as primary explosives, do not fit the characteristic of being "harder to initiate" in comparison to secondary explosives, as they typically deflagrate rather than detonate and can be initiated more readily than secondary explosives.