<human>: Objective: how to mix concrete for a slab

Which of the following solutions is more sound in terms of naive physics reasoning?

OPTIONS:
- A concrete mix of 1 part cement : 2 parts sand : 4 parts coarse aggregate should be used for a concrete slab. Ad water to proper consistency. Concrete must be placed after half an hour of mixing.
- A concrete mix of 1 part cement : 2 parts sand : 4 parts coarse aggregate should be used for a concrete slab. Ad water to proper consistency. Concrete must be placed within half an hour of mixing.
<bot>: A concrete mix of 1 part cement : 2 parts sand : 4 parts coarse aggregate should be used for a concrete slab. Ad water to proper consistency. Concrete must be placed within half an hour of mixing.