Which option identifies the substance that will accomplish the goals in the following scenario? A grower has been using Cornell Mix B (half peat, half perlite) as his soil-free growing medium, but has found that the growing system is causing much of the perlite to float to the top of the plug containers. The grower wishes to switch to something other than perlite to achieve a similar effect with a heavier substance that, like perlite, will still hold water. The replacement, however, must also provide good drainage to improve the drainage of the peat, which can retain too much water and drown the plugs.