Why is there a "b" in doubt? (Conjugate the verbs)

Gap-fill exercise

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
The present-day English word "doubt" (START) as a Latin word, "dubitare". It first (MOVE) from Latin into French where it (LOSE) both its "buh" sound and its letter "b". And then it (COME) into English in the 13th century.
About 100 years later, scribes who (WRITE) English but also (KNOW) Latin, (START) to reinsert the "b" into the word's spelling, even though no one (PRONOUNCE) it that way. But why they (? / DO) this? Why anyone in their right mind (? / REINSERT) a silent letter into a spelling?
Well, because they (KNOW) Latin, the scribes (UNDERSTAND) that the root of "doubt" (HAVE) a "b" in it. Over time, even as fewer literate people knew Latin, the "b" (KEEP) because it (MARK) important, meaningful connections to other related words...,