One of the main advantages of a graphite-furnace atomizer is that（）

A.It requires only very small quantities of a sample

B.It requires metal.ions in aqueous solution

C.It does not produce background emission like a flame

D.It does not get hot like a flame

岩溶地基采用大直径嵌岩桩，施工勘察时应逐桩布置勘探点。按《岩土工程勘察规范》（GB50021—2001）规定，勘探深度应不小于桩底以下3倍桩径，并不小于（）。

A.2m

B.3m

C.4m

D.5m