To carry out the task I should fill up the blanks with a correct verb form. Take a look at the given sentences:
"They feel sure they (arrest) the rest of the gang by the end of the month. This (is) the biggest art theft operation to be uncovered since 1974 when police (catch) a gang which (steal) over a million pounds worth of paintings from galleries all over the country."
There have been a lot of similar posts so far. I can't make up my mind which variant would be better to put down, though.
Would you mind to making sure if I have done it in a proper way?
"They feel sure they will have arrested (this task will be already done when there is the end of the month) the rest of the gang by the end of the month. This is (the operation is taking place at this time; different preparation are being made to catch the mobsters) / has been (we're talking about the experience of police since the stated year) the biggest art theft operation to be uncovered since 1974 when police caught (it's clear) a gang which had stolen (by 1974) over a million pounds worth of paintings from galleries all over the country."
I have doubts about ("is"/"has been"), but the whole passage needs to be looked through as well.