Automotive body engineering — when is frameless (unitized/monocoque) construction economical? Compared to a separate frame (body-on-frame), frameless or unitized construction becomes cost-effective when produced in __________ numbers.

Introduction / Context:
Automotive bodies may be built as body-on-frame or frameless (unitized/monocoque). Economic viability depends on tooling, manufacturing processes, and scale. This question probes production economics rather than structural behavior.

Given Data / Assumptions:

• Frameless structures need large body dies and extensive spot-welding/jig investments.
• Body-on-frame allows more flexibility with smaller tooling for low volume.

Concept / Approach:
High initial investment in press tools and body-in-white welding fixtures is amortized over many units. Therefore, the unit cost decreases markedly only when volume is high. Low volume cannot justify the tooling cost, favoring frame-based designs.

Step-by-Step Solution:

Verification / Alternative check:
Industry practice shows mass-market cars use unitized bodies, while niche trucks/buses and specialty vehicles often remain body-on-frame.

Why Other Options Are Wrong:

• Small or prototype runs (A/C/D): tooling cannot be recovered; per-unit cost is high.
• Seasonal batches (E): volume uncertainty still undermines tooling amortization.

Common Pitfalls:
Confusing structural efficiency (stiffness/weight) with production economics. A unitized body may be structurally superior yet not economical at tiny volumes.

Final Answer: