在检验某因素与某疾病的因果联系时，下列观察法最有效的是（）。

A.现况调查

B.生态学研究

C.病例报告

D.前瞻性队列研究

E.实验性研究

Compared to the prior year, Chart Industries has reported that its operating cycle has remained relatively stable while its cash conversion cycle has decreased. The most likely explanation for this is that the firm:（）

A. is relying more on its suppliers for short-term liquidity. 

B. is paying its bills for raw materials more rapidly. 

C. has improved its inventory turnover.