溃疡性结肠炎常见于（）

A．空肠或回肠 

B．回盲部 

C．直肠及乙状结肠 

D．左侧结肠 

E．末端回肠

In approximating costs, the estimator considers the possible causes of variation of the cost estimates, including .
A. budget B. plan C. risk D. contract