Write the next sentence in the following story.

How to make metal rust
Ensure that the metal you are working with will rust.
Only metals containing iron will rust, and some iron alloys will rust slowly or not at all. Stainless steel, an alloy of iron and chromium, will be very difficult to rust.

Choose your answer from:
- Check the rust conditions in your surface to make sure that it is not deteriorating. Also, be sure that the metal you are working with is one that can be cured, reoiled, sanded, burnt, or roasted.;
- Plan to work with stainless steel, unless you have any special requirements. Purchase a heavy-duty welding spanner.;
- Cast iron or wrought iron will rust most easily. Measure some hydrochloric acid into a plastic bottle.;
- Put oil or wd40 on the rust. Degrease it to get out.;. The answer should be
Cast iron or wrought iron will rust most easily. Measure some hydrochloric acid into a plastic bottle.