定义运算$a*b=\left\{\begin{array}{c}a\left(a\le b\right)\\ b\le a>b\end{array}\right.$则函数f（x）=1*2^x的最大值为____ 

下列说法中，正确的是（　　）

    British Airways booking secrets-and the best late deals this week: Short holidays and European city breaks.

    Dublin from $ 109 pp: Enjoy the Emerald Isle without the St Patrick's Day crowds. Departing 20th March, this package includes a two night stay at the four-star Kings Wood Hotel Citywest and return flights from London Stansted. Click here.

    Bruges from $ 159 pp: Treat mum to a mother's day city break in the medieval, fairly-tale town of Bruges and enjoy canal-side strolls and a nice helping of culture. Price includes two nights at the four-star NH Brugge with travel from London St Pancras, departing 20^th March. Click here.

    Madrid from $ 166 pp: Fly from Liverpool on March 27 for three nights' B& B at the three-star Espahotel Plaza de Espana. Real Madrid don't have a football game then, so sec if Ronaldo fancies a cerveza. Click here.

    French Alps from $ 499 pp: Save over $ 320 each on a week's skinning at Les Arcs 2000, staying catered at the Chalet Vanoise. Based on two sharing and includes flights from Manchester on March 18 and transfers. Six-day lift passes from $ 219, equipment hire from $ 65. Click here.

    Tenerife from $ 289 pp: Get your tan on early with a trip to the European home of beaches: Tenerife, with its postcard perfect golden sand. Spend seven nights at the four-star Hotel Spa La Quinta Park Suites, departing 6^th May and including return flights. Click here.