请教OG11-009（OG12-014）.语法区无人应答~

OG11-009（OG12-014）.Sunspots, vortices of gas associated with strong electromagnetic activity, are visible as dark spots on the surface of the Sun but have never been sighted on the Sun's poles or equator.

划线部分正确，但我觉得因为有but前后应该对称，前面是一般现在时后面的现在完成时为什么会对呢？

OG12-008

The widely accepted big bang theory holds that the universe began in an explosive instant ten to twenty billion years ago and has been expanding ever since.

同样，有and，但前后时态不一样也对？

刚开始复习，请xdjm指点下。

在语法区就像石沉大海一样,还没有一点浪花,特发到此板块,希望大家帮忙~