Furthermore, different rhinos leave different chemicals in their poop, which provide important 13 about the age, sex, general health, and reproductive conditions of each specific rhino.
答案：(D) clues
語意「提供關於年齡、性別……的重要『線索』」，clues（線索）符合這種透過化學物質推測資訊的語境；doubts（疑慮）、icons（圖示）、matches（比賽/火柴）都不合。
clue 你最熟的語境大概是推理小說或警匪片裡的「破案線索」，所以看到這裡在講化學物質，會覺得用 clue 很奇怪。其實 clue 指的是「可據以推論出答案的資訊」，不限犯罪場景，科學研究裡反而最常用它來指「推論的依據」。
空格後接 about the age, sex, general health, and reproductive conditions，介系詞 about 是關鍵——clue 的搭配正是 a clue to sth 或 a clue about sth。再看前一句 different rhinos leave different chemicals：化學物質本身不是答案，是讓其他犀牛「推論出」年齡與性別的依據，這就是 clue。
doubts 是疑慮，provide doubts about 語意不通，也與全文「糞便提供資訊」的正向論述相反；icons 你可能只想到手機桌面上的圖示，它另一個常考語意是「象徵性的代表人物、偶像」，兩個意思都放不進來；matches 是多義字，火柴／比賽／相配的人事物，三個意思在這裡都不合。
a clue to／about sth（……的線索）、provide／offer／give a clue（提供線索）、have no clue（完全不知道）、a match for sb（與某人匹敵的對手）、be a good match（很相配）。
From the campfire to the café, people have always gathered together to share the latest news. White rhinoceroses, 11 , do the same thing—only their choice of meeting place is a giant pile of poop. A new study indicates that rhinos have a smelly way to 12 .
Scientists studying white rhinos in South Africa have found that these animals leave messages in their poop. In particular, they leave their poop in the same place for all the other members in the group to smell—just like humans leaving messages on social media. Furthermore, different rhinos leave different chemicals in their poop, which provide important 13 about the age, sex, general health, and reproductive conditions of each specific rhino.
This is of great significance for the peace and harmony in the group. When all members in a group know what's going on with each other, it is 14 that they will fight.
The researchers have even made "fake poop" to see how different rhinos would react. They find that dominant males are particularly 15 the "fake poop" that carries chemicals from females ready to mate. The males sniff it for longer duration and come back to the same spot more frequently, a clear sign that they are gathering information about mating opportunities from the poop.