So stock market investors saw all this as good news for the worlds oil supply and prices decreased.

全球石油供应得以保持、油价下降的消息，的确令股票投资者倍感欣慰。

Average house prices decreased by 13% last year.

【decrease】去年房屋均价下降了 13%。

So, " to increase" means to go up - prices went up, prices increased; prices decreased.

所以，“to increase” 的意思是上涨——价格上涨，价格上涨； 价格下降。

When planes became more fuel-efficient, ticket prices decreased, and more people started to travel by plane.

当飞机变得越来越省油， 机票变得便宜， 也造成了更多人开始搭飞机四处旅游。

Last week, French supermarket Carrefour put stickers on products to warn consumers when a packet's contents have gotten smaller without a corresponding price decrease.

上周，法国超市家乐福在产品上贴上了标签，当一包产品的内容变少而价格却没有相应下降时，就会向消费者发出警告。