"Thank you dew" should also be accepted. It is weird, but how many Doulingo sentences are :)
It can be frustrating
The t sounds were probably different originally like Arabic ت andط (for one you put your tongue to the teeth, the other by putting the tongue on the roof of the mouth)
B/V as well as P/F and KH/K are actually the same sound in soft/hard versions where speech evolved over time to include the "hard" version, because starting a word with the soft, or saying it right after a full stop us harder. This makes these sounds grammatical, and not am actual part of the phonology.