Generate synonyms for the word 'challenging'.
difficult, demanding, tough, strenuous, formidable, arduous, laborious, testing, trying, knotty, problematic, tricky, onerous, taxing, exacting
Complete the following task based on the instruction and optional input provided.### Instruction:
Generate synonyms for the word 'challenging'.

### Output:
difficult, demanding, tough, strenuous, formidable, arduous, laborious, testing, trying, knotty, problematic, tricky, onerous, taxing, exacting