关于胎儿宫内发育迟缓的病因多而复杂，有些尚不明确，孕妇因素最常见，所占哪种比例是正确的：

A．90%

B．70%～80%

C．80%～90%

D．65%～70%

E．50%～60%

A municipal bond selling at 12% above par offers a yield of 3.2%. A taxable Treasury note selling at an 8% discount offers a yield of 4. 6%. An investor in the 32.5% tax bracket wishes to purchase an equal dollar amount of both bonds. The after-tax yield of the two-bond portfolio is closest to:（）

A. 2.63%.

B. 3.90%.

C. 3.15%.