The examiners know you might think "steep = difficult." So they will include another option that means difficult, without the rapid progress element.
If you are studying for this specific text (Test 7 in many practice books), these are the related terms often tested alongside it: FCE Context / Meaning A slope (used to contrast with "steep"). Mammoth task A huge or very difficult job. Agility Ability to move quickly and easily. With flying colours To pass or complete something very successfully. Get to grips with To begin to understand or deal with something difficult. Typical "Part 4" Transformation Answer a steep learning curve fce answers
She checked. No “at.” The original had “at the party,” but “since the party” was fine. Would they penalize? She didn’t know. The examiners know you might think "steep = difficult