问题
_______, when Marx was already _______, he found it was important to study the situation in Russia, so he began to learn Russian.
A:In 1870s, in his fifties
B:In 1870s, in fifties
C:In the 1870s, in fifties
D:In the 1870s, in his fifties;
B:In 1870s, in fifties
C:In the 1870s, in fifties
D:In the 1870s, in his fifties;
正确答案是：
