正确答案：BF
解析：
This is a tough question. The type of words that can fit in the blank is relatively obvious. Basically, officials are warning, urging, or recommending citizens. Now, we have to find a pair of words that create a similar meaning.
(A) is a neutral word that can work for the context. However, there is no matching word. (B), which means urging, works well, but apparently there are no matching words (we'll come back to that in a moment).
(C) is a word used by those not in a position of power to appeal to those in a position of power, which is the opposite of the context.
(D) works well. But it creates a meaning different from any of the other words. 'Enlisting' implies that the city is rounding up a group of specific people to help rake up leaves.
(E) means to recommend. This word is not quite the same as to remind.
That leaves us with (F), which is a tricky word. Enjoin can mean prohibit, which is clearly wrong in this context. Enjoin can also mean to urge, which is nearly the opposite of prohibit. In this sense, (F) and (B) create similar sentences.