Here's a problem to solve: Steel cans are separated from aluminum cans in a recycling center. Which of the following is the best way the recycling center can separate the steel cans from the aluminum cans?

Among the 4 following options, which is the correct answer?

- A: sort the cans by size
 
- B: put the cans in water
 
- C: cool the cans to a low temperature
 
- D: put the cans under an electromagnet
 