9700/11 Biology May Jun 2014 Online Test | Cambridge AS and A Level MCQ

In an investigation, discs of 2.5 cm diameter were cut from leaves. Fifty discs were put into three identical bags with different net sizes and buried in newly dug soil at a depth of 3cm.
At 60 day intervals the bags were dug up and the leaf discs measured to see how much of the leaf area had disappeared.
The table shows the percentage of leaf discs remaining in each bag.
Which organisms are most important for breaking down the leaves?