Energy security challenges for the 21st century Management challenges for the 21st century World population：challenges for the 21st century Management Challenges for the 21st Century Management challenges for the 21 century 并列题名：Management challenges for the 21st century Management Challenges for The 21st Century 公用事业管理：面对21世纪的挑战：challenges for the 21st century 动荡时代的企业责任：21世纪面临的挑战：challenges for the 21st century Engineering economics for the 21st century