It can be inferred from the passage that, in the early spring, the buds of plants ______

A．"export" protein in the form of amino acids

B．do not require water

C．have begun photosynthesizing

D．obtain organic compounds from the root

期货交易所会员、客户可以使用______等有价证券充抵保证金进行期货交易。

A．标准仓单

B．国债

C．股票

D．汇票