observant vs observing

To do their jobs well ,police officers must be very _________.

(1)observing
(2)observant
(3)observation
(4)to observe

書上的解答是
To do their jobs well ,police officers must be very observant.

不過我在想,能否選(1)observing???
"To do their jobs well ,police officers must be very observing"
我的想法是,這兩個選項都能當形容詞"觀察力敏銳的"

Yet a host of factors

Yet a host of factors pushes employers to hire fewer workers for more hours and, at the same time, compels workers to spend more time on the job

Yet=>然而
host=>許多

How fire leapt to life

"How and where man learnt how to produce flame at will is unknow"

這句話的意思因該是"人類學會生火昰在何時(何地)發生的,昰一個謎"
我的疑問昰為何用過去分詞"learnt"?而不是過去昰"learned"?