Соотнесите предложения 1 - 6 и союзы a - f. a)

Сопоставьте предложения 1 - 6 и союзы a - f. a) because b) how c) whose d) which e) what f) who 1) I've no idea ___ he has become a professional sportsman. 2) Ivan Kulibin, ___ Russians are proud of, was a great inventor. 3) We're very concerned ___ greenhouse effect means. 4) It seems very exciting to visit the London Tube, ___ is a modern railway system. 5) My son has no problems with his friends ___ he likes to get together. 6) Arthur Conan Doyle, ___ character Sherlock Holmes is well-known, was born in 1859.