Fill in the blank:
(a) The insects, butterflies, honeybees and birds help flowering plants in _______.
(b) A forest is a purifier of _______ and ________.
(c) Herbs form the ________ layer in the forest.
(d) The decaying leaves and animal droppings in a forest enrich the _______.
(a) The insects, butterflies, honeybees and birds help flowering plants in __pollination__.
(b) A forest is a purifier of __air__ and __water__.
(c) Herbs form the __lowest__ layer in the forest.
(d) The decaying leaves and animal droppings in a forest enrich the __soil__.