Enhancement of a word removal function in an existing C# software.

Current situation. I use a software to memorize texts: a few letters are removed in some selected words, and I have to add the first missing letter of each of these words. The level depends on the number of words to be removed, and the number of help letters given. Currently the same removal process is applied sentence after sentence, during a whole session. Expected enhancement. I want this level to change after each sentence, within the session. If all answers are good in sentence n, level is incremented for sentence n+1; if not, level goes down symetrically. Details --> Basic display: "Hell___ the worl___!"; expected answers: "o" then "d". --> Example of a sentence at level 4-3 (4 words removed and 3 help letters): One two thr___ fou___ fiv___ six sev___. --> Series of ordered levels: 9-2 (hardest), 8-2, 7-2, 6-2, 5-2, 4-2, 9-3, 8-3, 7-3, 6-3, 5-3, 4-3, 9-4, 8-4, 7-4, 6-4, 5-4, 4-4, 9-5, 8-5, 7-5, 6-5, 5-5, 4-5, 9-6, 8-6, 7-6, 6-6, 5-6, 4-6 (simplest). --> Next sentence if all answers have been good (level 5-3): Eight nine ten ele___ twe___ thirteen fou___ fifteen six___ sev___ eighteen.- --> Next sentence if not all answers have been good (level 9-4): Eigh___ nine ten elev___ twel___ thir___ four___ fift___ sixt___ seve___ eith___. Attachments - Source code. - Picture with typical parameters to start with. - Sample of a test text file. Delivery - You will send to me each beta version directly as an updated exe file so I can test them.