Question

if keys were made of glass, one problem would be __ they would break __

Explanation:

The problem is about the properties of glass as a material for keys. Glass is brittle, so a key made of glass would likely break easily when used, especially under the force needed to turn locks. The blank should be filled with a word or phrase that connects the cause (glass material) to the effect (breaking), like "that" to form "one problem would be that they would break".

Answer:

that