Read the following passage and answer the item that follows. Your answer to this item should be based on the passage only.

India's rivers are monitored at a few hundred fixed points, most of them near cities and most sampled once a month. A discharge that begins after the sampling van has left, or that enters a stretch between two stations, does not appear in the record at all. What the record therefore describes is not the state of the river but the state of the river at those places on those days. Continuous sensors are cheap enough now to be placed in far greater numbers, and where they have been installed the picture has changed sharply, with pollution events appearing that the monthly grab samples had never registered.

Based on the above passage, the following assumptions have been made:

1. Monthly sampling is adequate for rivers that do not flow past large cities.

2. The existing monitoring record understates how often pollution events occur.

3. Denser and more frequent measurement would give a more accurate picture of river quality.

4. Industries discharging into rivers deliberately time their releases to avoid the sampling van.

Which of the above assumptions are valid?