语法填空中遇到这样一个句子:
China speed is amazing the
world. While other countries are struggling to make high-speed railway a
reality, the China Railway
Corporation had busied /has been
busying itself in laying down over 25000 km of high-speed tracks over the past ten
years - accounting for roughly two thirds of the world's total length of
high-speed lines.
请教老师，这里为什么用过去完成时，过去完成时的使用不是要有明确的过去的时间做参照吗？
