What is the best word to replace the infinitive in the
sentence?
read the sentence
once the scientist completed to assemble the components
she tested her new device to see if it performed as
expected
assembling
assemble
assembled
assembly

well it is hard to say but everyone learns at there own pace  

i learn slow so it would take me like awhile

i hope i was so kinda

and if i was you in would goggle it and watch videos or go to your friends and ask them or maybe even your teacher for extra

answer is b,

"the tracer looks harmless in flight, but the pilot’s bitten lips and white knuckles suggest how dangerous it can be."

it was correct.