Questions

We know that, in solids all the particles are packed together and hence sound can travel from one to one very fast.

Hence, speed of sound is maximum in solids and decreses from solids to liquids and liquids to gases. Sound can't travel through vaccum.

Duge Beipanjiang Bridge in southwest China, opened to traffic this week and now the highest bridge in the world, some 565 metres (1,854 feet) above the valley below.The bridge stretches across a 1,341-metre (4,400-foot) span. Three years in the making, it's estimated to have cost the Chinese government around 1 billion yuan (or US$144 million).The bridge is going to cut travel times between Liupanshui in the Guizhou province and Xuanwei in the Yunnan province to two hours instead of the current five – as long as you don't mind the heights.The bridge crosses the the Nizhu river, and to give you some idea of the size of the drop down to the water below, it's almost 2 Eiffel Towers or one-and-a-half Empire State Buildings.

While moving from left to right in periodic table basic character of oxide of elements will decrease.

  ∴→Increasing basic strengthAl2O3<MgO<Na2O 

and while descending in the group basic character of corresponding oxides increases. 

 ∴→Increasing basic strengthNa2O<K2O

∴ Correct Order is Al2O3 < MgO <Na2O <K2O