[求助]几道语法题

1）Essential a nineteenth-century instrumen,,the caliope wasusually played outdoors and was a part of fairs, holiday gatherings, and other such acticities.

essential应怎么改？形容词改成副词，用副词来修饰一个名词短语，句首是形容词的不多见～

有两个形容词放句首的要小心，因为涉及到倒装，一个是typical of～ 一个是 characteristic of～

２)In geometry,an ellipse may be defined as the locus of all points,____distances from two  fixed point is constant.

A)which as the sum of

B)whose sum that the

ＡＢ怎么解释，我辨析不了．

首先明确是the sum of……是固定的，那么排除其它非the sum of ……的选项～

这里whose 修饰all points ，做定语从句，就是说两个固定点到它上面所有点的距离的和是常数～

如果用which 指代 all points，也就是which（all points） is constant，显然不符合原文the sum is constant 的意思～