None of them had any idea they were about to create a social innovation that 11 rapidly around the world.
答案：(B) would spread
過去未來式 would spread，站在1995年那個過去時間點，描述接下來將發生的事（他們沒想到這個創新後來會快速傳播到世界各地），是「過去看未來」的時態；had spread（過去完成式，時態方向不合，這是講「未來會發生」不是「已經發生過」）、had been spreading（同樣時態方向不合）、would have spread（過去完成的假設語氣，這裡不是假設）都不合。
句子裡有 None of them had any idea，你看到 had 就想「這裡要配過去完成式」，於是挑了 had spread。但 had any idea 的時態和空格裡的動詞是兩件事：had any idea 是 1995 年當下他們「不知道」，而不知道的內容是接下來會發生什麼。時間軸要分兩層：先有 1995 年這個過去的點，再有從那個點往後看的未來。
這是「站在過去看未來」的時態，文法上叫過去未來式，用 would ＋ 原形動詞。最快的判斷法是把它改回當年的直接說法：1995 年那天他們心裡若有數，想的會是 This will spread rapidly around the world.，把 will 往過去推一格就是 would spread。文內線索在最後一句 Since that rainy morning in Mill Valley, the World Café approach has been applied to...，擴散是 1995 年之後才發生的事，不可能在那之前就完成。
had spread 與 had been spreading 都是過去完成（進行）式，表示「在過去某個時間點之前就已經發生」，方向剛好相反。would have spread 是「本來會……（但實際沒有）」的假設語氣，可是世界咖啡館真的傳開了，不是與事實相反的假設——看到 would have Vpp，先問一句：這件事到底發生了沒有？
would ＋ V（過去未來式，「後來就會……」）、have no idea that...（完全不知道……）、spread rapidly／widely（迅速／廣泛傳播）、around the world（在世界各地）、would have Vpp（本來會……但實際沒有）。
In 1995, a group of business and academic leaders met at the home of Juanita Brown and David Isaacs in Mill Valley, California. None of them had any idea they were about to create a social innovation that 11 rapidly around the world.
The group was supposed to have a large-circle discussion in their beautiful garden. Unfortunately, it started pouring. With their plan 12 by the rain, the two dozen participants squeezed into the living room. They broke into small, intimate table conversations, recording their insights on paper tablecloths. They periodically stopped their conversations to switch tables so the insights and ideas might 13 and deepen. As they moved from one table to another, they noticed new ideas emerging from the discussions. This in turn enriched subsequent rounds of conversation.
Over the course of the morning, the innovative discussion process 14 a new form of collective effort that transformed the depth, scope, and quality of their discussion. The World Café was 15 created. Since that rainy morning in Mill Valley, the World Café approach has been applied to multi-group discussions and cooperative actions by businesses, industries, and educational institutions around the world.