The intersection of Cs and Ct is the set of lines on F that cuts the disjoint lines Ls and Lt. Consider the linear span of Ls and Lt : it is an hyperplane into P4 that cuts F into a smooth cubic surface.
ðɪ ˌɪntɚsˈɛkʃən ʌv sˌiːˈɛs ænd sˌiːtˈiː ɪz ðə sˈɛt ʌv lˈaɪnz ˌɔn ˈɛf ðæt kˈʌts ðə dɪsdʒˈɔɪnt lˈaɪnz ˌɛlˈɛs ænd ˌɛltˈiː. kənsˈɪdɚ ðə lˈɪnɪɹ spˈæn ʌv ˌɛlˈɛs ænd ˌɛltˈiː : ɪɾ ɪz æn hˌaɪpɚplˈeɪn ˌɪntʊ pˈiː fˈoːɹ ðæt kˈʌts ˈɛf ˌɪntʊ ɐ smˈuːð kjˈuːbɪk sˈɜːfɪs.