If you are invited to a party by the Austrians and the suggested dress is "informal " (usually marked in the lower left-hand coner of the invitation) , it may mean dark suit and a white shirt for men and cocktail dress( ) for women. Or again, may mean something considerably less formal. "Formal" means a black tie and evening dress for men and a floor-length dress for women. If a man is in doubt  about what to wear, he may come slightly overdressed and then take his coat and tie off when he sees that others are wearing sports shirts. National dress is always acceptable. Suppose you are a man invited to an informal party by an Austrian, you'd better  _  .
wear dark clothes and dark suit dress yourself in a dark suit and a white shirt wear a coat and so on wear sports shirts